diff --git a/src/Lean/Environment.lean b/src/Lean/Environment.lean
index 209c4ea06c..dc96222dc5 100644
--- a/src/Lean/Environment.lean
+++ b/src/Lean/Environment.lean
@@ -12,6 +12,13 @@ import Lean.Util.FindExpr
 import Lean.Util.Profile
 
 namespace Lean
+builtin_initialize importingRef : IO.Ref Bool ← IO.mkRef false
+
+/- True while modules are being imported. We use this flag to test check whether environment extensions are registered only
+   during initialization (builtin ones), and importing (user defined ones). -/
+def importing : IO Bool :=
+  importingRef.get
+
 /- Opaque environment extension state. -/
 constant EnvExtensionStateSpec : PointedType.{0}
 def EnvExtensionState : Type := EnvExtensionStateSpec.type
@@ -145,18 +152,20 @@ structure EnvExtensionInterface where
   registerExt  {σ} (mkInitial : IO σ) : IO (ext σ)
   setState     {σ} (e : ext σ) (env : Environment) : σ → Environment
   modifyState  {σ} (e : ext σ) (env : Environment) : (σ → σ) → Environment
-  getState     {σ} (e : ext σ) (env : Environment) : σ
+  getState     {σ} [Inhabited σ] (e : ext σ) (env : Environment) : σ
   mkInitialExtStates : IO (Array EnvExtensionState)
+  ensureExtensionsSize : Environment → IO Environment
 
 instance : Inhabited EnvExtensionInterface where
   default := {
-    ext                := id,
-    inhabitedExt       := id,
-    registerExt        := fun mk => mk,
-    setState           := fun _ env _ => env,
-    modifyState        := fun _ env _ => env,
-    getState           := fun ext _ => ext,
-    mkInitialExtStates := pure #[]
+    ext                  := id
+    inhabitedExt         := id
+    ensureExtensionsSize := fun env => pure env
+    registerExt          := fun mk => mk
+    setState             := fun _ env _ => env
+    modifyState          := fun _ env _ => env
+    getState             := fun ext _ => ext
+    mkInitialExtStates   := pure #[]
   }
 
 /- Unsafe implementation of `EnvExtensionInterface` -/
@@ -170,23 +179,53 @@ structure Ext (σ : Type) where
 private def mkEnvExtensionsRef : IO (IO.Ref (Array (Ext EnvExtensionState))) := IO.mkRef #[]
 @[builtinInit mkEnvExtensionsRef] private constant envExtensionsRef : IO.Ref (Array (Ext EnvExtensionState))
 
+/--
+  User-defined environment extensions are declared using the `initialize` command.
+  This command is just syntax sugar for the `init` attribute.
+  When we `import` lean modules, the vector stored at `envExtensionsRef` may increase in size because of
+  user-defined environment extensions. When this happens, we must adjust the size of the `env.extensions`.
+  This method is invoked when processing `import`s.
+-/
+partial def ensureExtensionsArraySize (env : Environment) : IO Environment := do
+  loop env.extensions.size env
+where
+  loop (i : Nat) (env : Environment) : IO Environment := do
+    let envExtensions ← envExtensionsRef.get
+    if h : i < envExtensions.size then
+      let s ← envExtensions[i].mkInitial
+      let env := { env with extensions := env.extensions.push s }
+      loop (i + 1) env
+    else
+      return env
+
+private def invalidExtMsg := "invalid environment extension has been accessed"
+
 unsafe def setState {σ} (ext : Ext σ) (env : Environment) (s : σ) : Environment :=
-  { env with extensions := env.extensions.set! ext.idx (unsafeCast s) }
+  if h : ext.idx < env.extensions.size then
+    { env with extensions := env.extensions.set ⟨ext.idx, h⟩ (unsafeCast s) }
+  else
+    panic! invalidExtMsg
 
 @[inline] unsafe def modifyState {σ : Type} (ext : Ext σ) (env : Environment) (f : σ → σ) : Environment :=
-  { env with
-    extensions := env.extensions.modify ext.idx fun s =>
-      let s : σ := unsafeCast s;
-      let s : σ := f s;
-      unsafeCast s }
+  if ext.idx < env.extensions.size then
+    { env with
+      extensions := env.extensions.modify ext.idx fun s =>
+        let s : σ := unsafeCast s
+        let s : σ := f s
+        unsafeCast s }
+  else
+    panic! invalidExtMsg
 
-unsafe def getState {σ} (ext : Ext σ) (env : Environment) : σ :=
-  let s : EnvExtensionState := env.extensions.get! ext.idx
-  unsafeCast s
+unsafe def getState {σ} [Inhabited σ] (ext : Ext σ) (env : Environment) : σ :=
+  if h : ext.idx < env.extensions.size then
+    let s : EnvExtensionState := env.extensions.get ⟨ext.idx, h⟩
+    unsafeCast s
+  else
+    panic! invalidExtMsg
 
 unsafe def registerExt {σ} (mkInitial : IO σ) : IO (Ext σ) := do
-  let initializing ← IO.initializing
-  unless initializing do throw (IO.userError "failed to register environment, extensions can only be registered during initialization")
+  unless (← IO.initializing) || (← importing) do
+    throw (IO.userError "failed to register environment, extensions can only be registered during initialization")
   let exts ← envExtensionsRef.get
   let idx := exts.size
   let ext : Ext σ := {
@@ -201,13 +240,14 @@ def mkInitialExtStates : IO (Array EnvExtensionState) := do
   exts.mapM fun ext => ext.mkInitial
 
 unsafe def imp : EnvExtensionInterface := {
-  ext                := Ext,
-  inhabitedExt       := fun _ => ⟨arbitrary⟩,
-  registerExt        := registerExt,
-  setState           := setState,
-  modifyState        := modifyState,
-  getState           := getState,
-  mkInitialExtStates := mkInitialExtStates
+  ext                  := Ext
+  ensureExtensionsSize := ensureExtensionsArraySize
+  inhabitedExt         := fun _ => ⟨arbitrary⟩
+  registerExt          := registerExt
+  setState             := setState
+  modifyState          := modifyState
+  getState             := getState
+  mkInitialExtStates   := mkInitialExtStates
 }
 
 end EnvExtensionInterfaceUnsafe
@@ -217,11 +257,14 @@ constant EnvExtensionInterfaceImp : EnvExtensionInterface
 
 def EnvExtension (σ : Type) : Type := EnvExtensionInterfaceImp.ext σ
 
+private def ensureExtensionsArraySize (env : Environment) : IO Environment :=
+  EnvExtensionInterfaceImp.ensureExtensionsSize env
+
 namespace EnvExtension
 instance {σ} [s : Inhabited σ] : Inhabited (EnvExtension σ) := EnvExtensionInterfaceImp.inhabitedExt s
 def setState {σ : Type} (ext : EnvExtension σ) (env : Environment) (s : σ) : Environment := EnvExtensionInterfaceImp.setState ext env s
 def modifyState {σ : Type} (ext : EnvExtension σ) (env : Environment) (f : σ → σ) : Environment := EnvExtensionInterfaceImp.modifyState ext env f
-def getState {σ : Type} (ext : EnvExtension σ) (env : Environment) : σ := EnvExtensionInterfaceImp.getState ext env
+def getState {σ : Type} [Inhabited σ] (ext : EnvExtension σ) (env : Environment) : σ := EnvExtensionInterfaceImp.getState ext env
 end EnvExtension
 
 /- Environment extensions can only be registered during initialization.
@@ -297,7 +340,7 @@ instance {α β σ} [Inhabited σ] : Inhabited (PersistentEnvExtension α β σ)
 
 namespace PersistentEnvExtension
 
-def getModuleEntries {α β σ : Type} (ext : PersistentEnvExtension α β σ) (env : Environment) (m : ModuleIdx) : Array α :=
+def getModuleEntries {α β σ : Type} [Inhabited σ] (ext : PersistentEnvExtension α β σ) (env : Environment) (m : ModuleIdx) : Array α :=
   (ext.toEnvExtension.getState env).importedEntries.get! m
 
 def addEntry {α β σ : Type} (ext : PersistentEnvExtension α β σ) (env : Environment) (b : β) : Environment :=
@@ -305,7 +348,7 @@ def addEntry {α β σ : Type} (ext : PersistentEnvExtension α β σ) (env : En
     let state   := ext.addEntryFn s.state b;
     { s with state := state }
 
-def getState {α β σ : Type} (ext : PersistentEnvExtension α β σ) (env : Environment) : σ :=
+def getState {α β σ : Type} [Inhabited σ] (ext : PersistentEnvExtension α β σ) (env : Environment) : σ :=
   (ext.toEnvExtension.getState env).state
 
 def setState {α β σ : Type} (ext : PersistentEnvExtension α β σ) (env : Environment) (s : σ) : Environment :=
@@ -379,10 +422,10 @@ namespace SimplePersistentEnvExtension
 instance {α σ : Type} [Inhabited σ] : Inhabited (SimplePersistentEnvExtension α σ) :=
   inferInstanceAs (Inhabited (PersistentEnvExtension α α (List α × σ)))
 
-def getEntries {α σ : Type} (ext : SimplePersistentEnvExtension α σ) (env : Environment) : List α :=
+def getEntries {α σ : Type} [Inhabited σ] (ext : SimplePersistentEnvExtension α σ) (env : Environment) : List α :=
   (PersistentEnvExtension.getState ext env).1
 
-def getState {α σ : Type} (ext : SimplePersistentEnvExtension α σ) (env : Environment) : σ :=
+def getState {α σ : Type} [Inhabited σ] (ext : SimplePersistentEnvExtension α σ) (env : Environment) : σ :=
   (PersistentEnvExtension.getState ext env).2
 
 def setState {α σ : Type} (ext : SimplePersistentEnvExtension α σ) (env : Environment) (s : σ) : Environment :=
@@ -518,23 +561,36 @@ private partial def getEntriesFor (mod : ModuleData) (extId : Name) (i : Nat) :
   else
     #[]
 
-private def setImportedEntries (env : Environment) (mods : Array ModuleData) : IO Environment := do
+private def setImportedEntries (env : Environment) (mods : Array ModuleData) (startingAt : Nat := 0) : IO Environment := do
   let mut env := env
   let pExtDescrs ← persistentEnvExtensionsRef.get
   for mod in mods do
-    for extDescr in pExtDescrs do
+    for extDescr in pExtDescrs[startingAt:] do
       let entries := getEntriesFor mod extDescr.name 0
       env ← extDescr.toEnvExtension.modifyState env fun s => { s with importedEntries := s.importedEntries.push entries }
   return env
 
-private def finalizePersistentExtensions (env : Environment) (opts : Options) : IO Environment := do
-  let mut env := env
-  let pExtDescrs ← persistentEnvExtensionsRef.get
-  for extDescr in pExtDescrs do
-    let s := extDescr.toEnvExtension.getState env
-    let newState ← extDescr.addImportedFn s.importedEntries { env := env, opts := opts }
-    env ← extDescr.toEnvExtension.setState env { s with state := newState }
-  return env
+private partial def finalizePersistentExtensions (env : Environment) (mods : Array ModuleData) (opts : Options) : IO Environment := do
+  loop 0 env
+where
+  loop (i : Nat) (env : Environment) : IO Environment := do
+    -- Recall that the size of the array stored `persistentEnvExtensionRef` may increase when we import user-defined environment extensions.
+    let pExtDescrs ← persistentEnvExtensionsRef.get
+    if h : i < pExtDescrs.size then
+      let extDescr := pExtDescrs[i]
+      let s := extDescr.toEnvExtension.getState env
+      let prevSize := (← persistentEnvExtensionsRef.get).size
+      let newState ← extDescr.addImportedFn s.importedEntries { env := env, opts := opts }
+      let mut env ← extDescr.toEnvExtension.setState env { s with state := newState }
+      env ← ensureExtensionsArraySize env
+      if (← persistentEnvExtensionsRef.get).size > prevSize then
+        -- This branch is executed when `pExtDescrs[i]` is the extension associated with the `init` attribute, and
+        -- a user-defined persistent extension is imported.
+        -- Thus, we invoke `setImportedEntries` to update the array `importedEntries` with the entries for the new extensions.
+        env ← setImportedEntries env mods prevSize
+      loop (i + 1) env
+    else
+      return env
 
 structure ImportState where
   moduleNameSet : NameSet := {}
@@ -544,34 +600,38 @@ structure ImportState where
 
 @[export lean_import_modules]
 partial def importModules (imports : List Import) (opts : Options) (trustLevel : UInt32 := 0) : IO Environment := profileitIO "import" opts do
-  let (_, s) ← importMods imports |>.run {}
-  -- (moduleNames, mods, regions)
-  let mut modIdx : Nat := 0
-  let mut const2ModIdx : HashMap Name ModuleIdx := {}
-  let mut constants : ConstMap := SMap.empty
-  for mod in s.moduleData do
-    for cinfo in mod.constants do
-      const2ModIdx := const2ModIdx.insert cinfo.name modIdx
-      if constants.contains cinfo.name then throw (IO.userError s!"import failed, environment already contains '{cinfo.name}'")
-      constants := constants.insert cinfo.name cinfo
-    modIdx := modIdx + 1
-  constants   := constants.switch
-  let exts ← mkInitialExtensionStates
-  let env : Environment := {
-    const2ModIdx := const2ModIdx,
-    constants    := constants,
-    extensions   := exts,
-    header       := {
-      quotInit     := !imports.isEmpty, -- We assume `core.lean` initializes quotient module
-      trustLevel   := trustLevel,
-      imports      := imports.toArray,
-      regions      := s.regions,
-      moduleNames  := s.moduleNames
+  try
+    importingRef.set true
+    let (_, s) ← importMods imports |>.run {}
+    -- (moduleNames, mods, regions)
+    let mut modIdx : Nat := 0
+    let mut const2ModIdx : HashMap Name ModuleIdx := {}
+    let mut constants : ConstMap := SMap.empty
+    for mod in s.moduleData do
+      for cinfo in mod.constants do
+        const2ModIdx := const2ModIdx.insert cinfo.name modIdx
+        if constants.contains cinfo.name then throw (IO.userError s!"import failed, environment already contains '{cinfo.name}'")
+        constants := constants.insert cinfo.name cinfo
+      modIdx := modIdx + 1
+    constants   := constants.switch
+    let exts ← mkInitialExtensionStates
+    let env : Environment := {
+      const2ModIdx := const2ModIdx,
+      constants    := constants,
+      extensions   := exts,
+      header       := {
+        quotInit     := !imports.isEmpty, -- We assume `core.lean` initializes quotient module
+        trustLevel   := trustLevel,
+        imports      := imports.toArray,
+        regions      := s.regions,
+        moduleNames  := s.moduleNames
+      }
     }
-  }
-  let env ← setImportedEntries env s.moduleData
-  let env ← finalizePersistentExtensions env opts
-  pure env
+    let env ← setImportedEntries env s.moduleData
+    let env ← finalizePersistentExtensions env s.moduleData opts
+    pure env
+  finally
+    importingRef.set false
 where
   importMods : List Import → StateRefT ImportState IO Unit
   | []    => pure ()
diff --git a/src/shell/CMakeLists.txt b/src/shell/CMakeLists.txt
index d68e1513c3..4949a8729a 100644
--- a/src/shell/CMakeLists.txt
+++ b/src/shell/CMakeLists.txt
@@ -179,3 +179,11 @@ add_test(NAME leanpkgtest_cyclic
            set -eu
            export PATH=${LEAN_BIN}:$PATH
            leanpkg build 2>&1 | grep 'import cycle'")
+
+add_test(NAME leanpkgtest_user_ext
+         WORKING_DIRECTORY "${LEAN_SOURCE_DIR}/../tests/leanpkg/user_ext"
+         COMMAND bash -c "
+           set -eu
+           export PATH=${LEAN_BIN}:$PATH
+           find . -name '*.olean' -delete
+           leanpkg build | grep 'world, hello, test'")
diff --git a/tests/leanpkg/user_ext/.gitignore b/tests/leanpkg/user_ext/.gitignore
new file mode 100644
index 0000000000..796b96d1c4
--- /dev/null
+++ b/tests/leanpkg/user_ext/.gitignore
@@ -0,0 +1 @@
+/build
diff --git a/tests/leanpkg/user_ext/UserExt.lean b/tests/leanpkg/user_ext/UserExt.lean
new file mode 100644
index 0000000000..b8531c360c
--- /dev/null
+++ b/tests/leanpkg/user_ext/UserExt.lean
@@ -0,0 +1,5 @@
+import UserExt.Tst1
+import UserExt.Tst2
+
+show_foo_set
+show_bla_set
diff --git a/tests/leanpkg/user_ext/UserExt/BlaExt.lean b/tests/leanpkg/user_ext/UserExt/BlaExt.lean
new file mode 100644
index 0000000000..5284117fe9
--- /dev/null
+++ b/tests/leanpkg/user_ext/UserExt/BlaExt.lean
@@ -0,0 +1,23 @@
+import Lean
+
+open Lean
+
+initialize blaExtension : SimplePersistentEnvExtension Name NameSet ←
+  registerSimplePersistentEnvExtension {
+    name          := `blaExt
+    addEntryFn    := NameSet.insert
+    addImportedFn := fun es => mkStateFromImportedEntries NameSet.insert {} es
+  }
+
+syntax (name := insertBla) "insert_bla " ident : command
+syntax (name := showBla) "show_bla_set" : command
+
+open Lean.Elab
+open Lean.Elab.Command
+
+@[commandElab insertBla] def elabInsertBla : CommandElab := fun stx => do
+  IO.println s!"inserting {stx[1].getId}"
+  modifyEnv fun env => blaExtension.addEntry env stx[1].getId
+
+@[commandElab showBla] def elabShowBla : CommandElab := fun stx => do
+  IO.println s!"bla set: {blaExtension.getState (← getEnv) |>.toList}"
diff --git a/tests/leanpkg/user_ext/UserExt/FooExt.lean b/tests/leanpkg/user_ext/UserExt/FooExt.lean
new file mode 100644
index 0000000000..6618dbd4a9
--- /dev/null
+++ b/tests/leanpkg/user_ext/UserExt/FooExt.lean
@@ -0,0 +1,23 @@
+import Lean
+
+open Lean
+
+initialize fooExtension : SimplePersistentEnvExtension Name NameSet ←
+  registerSimplePersistentEnvExtension {
+    name          := `fooExt
+    addEntryFn    := NameSet.insert
+    addImportedFn := fun es => mkStateFromImportedEntries NameSet.insert {} es
+  }
+
+syntax (name := insertFoo) "insert_foo " ident : command
+syntax (name := showFoo) "show_foo_set" : command
+
+open Lean.Elab
+open Lean.Elab.Command
+
+@[commandElab insertFoo] def elabInsertFoo : CommandElab := fun stx => do
+  IO.println s!"inserting {stx[1].getId}"
+  modifyEnv fun env => fooExtension.addEntry env stx[1].getId
+
+@[commandElab showFoo] def elabShowFoo : CommandElab := fun stx => do
+  IO.println s!"foo set: {fooExtension.getState (← getEnv) |>.toList}"
diff --git a/tests/leanpkg/user_ext/UserExt/Tst1.lean b/tests/leanpkg/user_ext/UserExt/Tst1.lean
new file mode 100644
index 0000000000..df3f4ff5ce
--- /dev/null
+++ b/tests/leanpkg/user_ext/UserExt/Tst1.lean
@@ -0,0 +1,9 @@
+import UserExt.FooExt
+import UserExt.BlaExt
+
+insert_foo hello
+insert_foo world
+show_foo_set
+
+insert_bla abc
+show_bla_set
diff --git a/tests/leanpkg/user_ext/UserExt/Tst2.lean b/tests/leanpkg/user_ext/UserExt/Tst2.lean
new file mode 100644
index 0000000000..c7743d2044
--- /dev/null
+++ b/tests/leanpkg/user_ext/UserExt/Tst2.lean
@@ -0,0 +1,6 @@
+import UserExt.BlaExt
+import UserExt.FooExt
+
+insert_foo test
+insert_bla defg
+insert_bla hij
diff --git a/tests/leanpkg/user_ext/leanpkg.toml b/tests/leanpkg/user_ext/leanpkg.toml
new file mode 100644
index 0000000000..2606dd26d3
--- /dev/null
+++ b/tests/leanpkg/user_ext/leanpkg.toml
@@ -0,0 +1,3 @@
+[package]
+name = "UserExt"
+version = "0.1"