diff --git a/lean4-mode/lean4-syntax.el b/lean4-mode/lean4-syntax.el
index 9bcbe95ffd..a2d3a90018 100644
--- a/lean4-mode/lean4-syntax.el
+++ b/lean4-mode/lean4-syntax.el
@@ -17,7 +17,7 @@
     "end" "this" "using" "using_well_founded" "namespace" "section"
     "attribute" "local" "set_option" "extends" "include" "class"
     "attributes" "raw" "have" "show" "suffices" "by" "in" "at" "do" "let" "for" "unless" "break" "continue"
-    "try" "catch" "finally" "where" "rec" "forall" "fun"
+    "try" "catch" "finally" "where" "rec" "mut" "forall" "fun"
     "exists" "if" "then" "else" "from" "init_quot" "return"
     "mutual" "def" "run_cmd" "declare_syntax_cat" "syntax" "macro_rules" "macro"
     "initialize" "builtin_initialize")
diff --git a/src/Lean/Elab/Do.lean b/src/Lean/Elab/Do.lean
index ac7214ee22..3959c7e00f 100644
--- a/src/Lean/Elab/Do.lean
+++ b/src/Lean/Elab/Do.lean
@@ -1097,37 +1097,32 @@ def matchNestedTermResult (ref : Syntax) (term : Syntax) (uvars : Array Name) (a
 
 end ToTerm
 
+def isMutableLet (doElem : Syntax) : Bool :=
+  let kind := doElem.getKind
+  (kind == `Lean.Parser.Term.doLetArrow || kind == `Lean.Parser.Term.doLet)
+  &&
+  !doElem[1].isNone
+
 namespace ToCodeBlock
 
 structure Context :=
-  (ref       : Syntax)
-  (m         : Syntax) -- Syntax representing the monad associated with the do notation.
-  (varSet    : NameSet := {})
-  (insideFor : Bool := false)
+  (ref         : Syntax)
+  (m           : Syntax) -- Syntax representing the monad associated with the do notation.
+  (mutableVars : NameSet := {})
+  (insideFor   : Bool := false)
 
 abbrev M := ReaderT Context TermElabM
 
-@[inline] def withNewVars {α} (newVars : Array Name) (x : M α) : M α :=
-  withReader (fun ctx => { ctx with varSet := insertVars ctx.varSet newVars }) x
-
-builtin_initialize
-  registerOption `relaxedReassignments { defValue := false, group := "do", descr := "if set to true, then any variable in the local context may be reassigned" }
-
-def getRelaxedReassigments : M Bool := do
-  return (← getOptions).get `relaxedReassignments false
+@[inline] def withNewMutableVars {α} (newVars : Array Name) (mutable : Bool) (x : M α) : M α :=
+  withReader (fun ctx => if mutable then { ctx with mutableVars := insertVars ctx.mutableVars newVars } else ctx) x
 
 def checkReassignable (xs : Array Name) : M Unit := do
   let throwInvalidReassignment (x : Name) : M Unit :=
     throwError! "'{x.simpMacroScopes}' cannot be reassigned"
   let ctx ← read
   for x in xs do
-    unless ctx.varSet.contains x do
-      if (← getRelaxedReassigments) then
-        match (← resolveLocalName x) with
-        | some (_, []) => pure ()
-        | _ => throwInvalidReassignment x
-      else
-        throwInvalidReassignment x
+    unless ctx.mutableVars.contains x do
+      throwInvalidReassignment x
 
 @[inline] def withFor {α} (x : M α) : M α :=
   withReader (fun ctx => { ctx with insideFor := true }) x
@@ -1203,12 +1198,12 @@ def checkLetArrowRHS (doElem : Syntax) : M Unit := do
    def doPatDecl  := parser! termParser >> leftArrow >> doElemParser >> optional (" | " >> doElemParser)
    ``` -/
 def doLetArrowToCode (doSeqToCode : List Syntax → M CodeBlock) (doLetArrow : Syntax) (doElems : List Syntax) : M CodeBlock := do
-  let ref  := doLetArrow
-  let decl := doLetArrow[2]
+  let ref     := doLetArrow
+  let decl    := doLetArrow[2]
   if decl.getKind == `Lean.Parser.Term.doIdDecl then
     let y := decl[0].getId
     let doElem := decl[3]
-    let k ← withNewVars #[y] (doSeqToCode doElems)
+    let k ← withNewMutableVars #[y] (isMutableLet doLetArrow) (doSeqToCode doElems)
     match isDoExpr? doElem with
     | some action => pure $ mkVarDeclCore #[y] doLetArrow k
     | none =>
@@ -1222,9 +1217,15 @@ def doLetArrowToCode (doSeqToCode : List Syntax → M CodeBlock) (doLetArrow : S
     let doElem  := decl[2]
     let optElse := decl[3]
     if optElse.isNone then withFreshMacroScope do
-      let auxDo ← `(do let discr ← $doElem; let $pattern:term := discr)
+      let auxDo ←
+        if isMutableLet doLetArrow then
+          `(do let discr ← $doElem; let mut $pattern:term := discr)
+        else
+          `(do let discr ← $doElem; let $pattern:term := discr)
       doSeqToCode $ getDoSeqElems (getDoSeq auxDo) ++ doElems
     else
+      if isMutableLet doLetArrow then
+        throwError! "'mut' is currently not supported in let-decls with 'else' case"
       let contSeq := mkDoSeq doElems.toArray
       let elseSeq := mkSingletonDoSeq optElse[1]
       let auxDo ← `(do let discr ← $doElem; match discr with | $pattern:term => $contSeq | _ => $elseSeq)
@@ -1295,8 +1296,7 @@ def doForToCode (doSeqToCode : List Syntax → M CodeBlock) (doFor : Syntax) (do
   let x         := doFor[1]
   let xs        := doFor[3]
   let forElems  := getDoSeqElems doFor[5]
-  let newVars   := if x.isIdent then #[x.getId] else #[]
-  let forInBodyCodeBlock  ← withNewVars newVars $ withFor (doSeqToCode forElems)
+  let forInBodyCodeBlock ← withFor (doSeqToCode forElems)
   let ⟨uvars, forInBody⟩ ← mkForInBody x forInBodyCodeBlock
   let uvarsTuple ← liftMacroM $ mkTuple ref (uvars.map (mkIdentFrom ref))
   if hasReturn forInBodyCodeBlock.code then
@@ -1335,7 +1335,7 @@ def doMatchToCode (doSeqToCode : List Syntax → M CodeBlock) (doMatch : Syntax)
     let pvars ← getPatternsVars patterns.getSepArgs
     let vars := getPatternVarNames pvars
     let rhs  := matchAlt[2]
-    let rhs ← withNewVars vars $ doSeqToCode (getDoSeqElems rhs)
+    let rhs ← doSeqToCode (getDoSeqElems rhs)
     pure { ref := matchAlt, vars := vars, patterns := patterns, rhs := rhs : Alt CodeBlock }
   let matchCode ← mkMatch ref discrs optType alts
   concatWith doSeqToCode matchCode doElems
@@ -1450,13 +1450,13 @@ partial def doSeqToCode : List Syntax → M CodeBlock
         let k := doElem.getKind
         if k == `Lean.Parser.Term.doLet then
           let vars ← getDoLetVars doElem
-          mkVarDeclCore vars doElem <$> withNewVars vars (doSeqToCode doElems)
+          mkVarDeclCore vars doElem <$> withNewMutableVars vars (isMutableLet doElem) (doSeqToCode doElems)
         else if k == `Lean.Parser.Term.doHave then
           let var := getDoHaveVar doElem
-          mkVarDeclCore #[var] doElem <$> withNewVars #[var] (doSeqToCode doElems)
+          mkVarDeclCore #[var] doElem <$> (doSeqToCode doElems)
         else if k == `Lean.Parser.Term.doLetRec then
           let vars ← getDoLetRecVars doElem
-          mkVarDeclCore vars doElem <$> withNewVars vars (doSeqToCode doElems)
+          mkVarDeclCore vars doElem <$> (doSeqToCode doElems)
         else if k == `Lean.Parser.Term.doReassign then
           let vars ← liftM $ getDoReassignVars doElem
           checkReassignable vars
diff --git a/tests/compiler/rbmap_library.lean b/tests/compiler/rbmap_library.lean
index 630729d65b..2a2f86e748 100644
--- a/tests/compiler/rbmap_library.lean
+++ b/tests/compiler/rbmap_library.lean
@@ -53,11 +53,11 @@ def mkRandMap (max : Nat) : Nat → Map → Array (Nat × Nat) → IO (Map × Ar
 
 def tst3 (seed : Nat) (n : Nat) (max : Nat) : IO Unit :=
 do IO.setRandSeed seed
-   let (m, a) ← mkRandMap max n {} Array.empty
+   let mut (m, a) ← mkRandMap max n {} Array.empty
    check (sz m == a.size)
    check (a.all (fun ⟨k, v⟩ => m.find? k == some v))
    IO.println ("tst3 size: " ++ toString a.size)
-   let i := 0
+   let mut i := 0
    for (k, b) in a do
      if i % 2 == 0 then
        m := m.erase k
diff --git a/tests/lean/doNotation1.lean b/tests/lean/doNotation1.lean
index 11a3131ced..6f7f956a93 100644
--- a/tests/lean/doNotation1.lean
+++ b/tests/lean/doNotation1.lean
@@ -14,13 +14,13 @@ for p in xs do
 inductive Vector (α : Type) : Nat → Type
 | nil  : Vector α 0
 | cons : α → {n : Nat} → Vector α n → Vector α (n+1)
-set_option relaxedReassignments true in
 def f4 (b : Bool) (n : Nat) (v : Vector Nat n) : Vector Nat (n+1) := do
+let mut v := v
 if b then
   v := Vector.cons 1 v
 Vector.cons 1 v
-set_option relaxedReassignments true in
 def f5 (y : Nat) (xs : List Nat) : List Bool := do
+let mut y := y
 for x in xs do
   y := true -- invalid reassigned
 
@@ -49,8 +49,8 @@ def f11 (x : Nat) : IO Unit := do
 if x > 0 then
   IO.println "x is not zero"
 IO.mkRef true -- error here as expected
-set_option relaxedReassignments true in
 def f12 (x : Nat) : IO Unit := do
+let mut x := x
 if x > 0 then
   pure true
 else
diff --git a/tests/lean/doNotation1.lean.expected.out b/tests/lean/doNotation1.lean.expected.out
index 486d3af8ea..6cc895af87 100644
--- a/tests/lean/doNotation1.lean.expected.out
+++ b/tests/lean/doNotation1.lean.expected.out
@@ -1,6 +1,6 @@
 doNotation1.lean:4:0: error: 'y' cannot be reassigned
 doNotation1.lean:8:2: error: 'y' cannot be reassigned
-doNotation1.lean:11:0: error: 'p' cannot be reassigned
+doNotation1.lean:12:2: error: 'p' cannot be reassigned
 doNotation1.lean:20:7: error: invalid reassignment, value has type
   Vector Nat (n + 1)
 but is expected to have type
diff --git a/tests/lean/run/depElim1.lean b/tests/lean/run/depElim1.lean
index e827072d11..e6dd22639b 100644
--- a/tests/lean/run/depElim1.lean
+++ b/tests/lean/run/depElim1.lean
@@ -139,7 +139,7 @@ registerTraceClass `Meta.mkElim
 /- Helper methods for testins mkElim -/
 
 private def getUnusedLevelParam (majors : List Expr) (lhss : List AltLHS) : MetaM Level := do
-let s := {}
+let mut s := {}
 for major in majors do
   let major ← instantiateMVars major
   let majorType ← inferType major
diff --git a/tests/lean/run/doNotation1.lean b/tests/lean/run/doNotation1.lean
index 5fb625986f..a33690d6fe 100644
--- a/tests/lean/run/doNotation1.lean
+++ b/tests/lean/run/doNotation1.lean
@@ -1,4 +1,3 @@
-
 open Lean
 
 def f : IO Nat :=
@@ -122,8 +121,8 @@ else
 
 def f1 (x : Nat) : StateT Nat IO Nat := do
 IO.println "hello"
-let z := x
-let y := x
+let mut z := x
+let mut y := x
 modify (· + 10)
 if x > 0 then
   y := 3*y
diff --git a/tests/lean/run/doNotation2.lean b/tests/lean/run/doNotation2.lean
index d69bc8a8c3..684f0f4608 100644
--- a/tests/lean/run/doNotation2.lean
+++ b/tests/lean/run/doNotation2.lean
@@ -15,7 +15,7 @@ aux x x;
 #eval f 10
 
 def g (xs : List Nat) : StateT Nat Id Nat := do
-let xs := xs
+let mut xs := xs
 if xs.isEmpty then
   xs := [← get]
 dbgTrace! ">>> xs: {xs}"
@@ -31,8 +31,8 @@ theorem ex2 : (g [] $.run' 0) = 1 :=
 rfl
 
 def h (x : Nat) (y : Nat) : Nat := do
-let x := x
-let y := y
+let mut x := x
+let mut y := y
 if x > 0 then
   let y := x + 1 -- this is a new `y` that shadows the one above
   x := y
@@ -47,7 +47,7 @@ theorem ex4 (y : Nat) : h 1 y = (1 + 1) + y :=
 rfl
 
 def sumOdd (xs : List Nat) (threshold : Nat) : Nat := do
-let sum := 0
+let mut sum := 0
 for x in xs do
   if x % 2 == 1 then
     sum := sum + x
@@ -65,7 +65,7 @@ rfl
 
 -- We need `Id.run` because we still have `Monad Option`
 def find? (xs : List Nat) (p : Nat → Bool) : Option Nat := Id.run do
-let result := none
+let mut result := none
 for x in xs do
   if p x then
     result := x
@@ -73,7 +73,7 @@ for x in xs do
 return result
 
 def sumDiff (ps : List (Nat × Nat)) : Nat := do
-let sum := 0
+let mut sum := 0
 for (x, y) in ps do
   sum := sum + x - y
 return sum
@@ -103,8 +103,8 @@ IO.println ("isOdd(" ++ toString x ++ "): " ++ toString (isOdd x))
 #eval f2 10
 
 def split (xs : List Nat) : List Nat × List Nat := do
-let evens := []
-let odds  := []
+let mut evens := []
+let mut odds  := []
 for x in xs.reverse do
   if x % 2 == 0 then
     evens := x :: evens
@@ -119,12 +119,12 @@ def f3 (x : Nat) : IO Bool := do
 let y ← cond (x == 0) (do IO.println "hello"; true) false;
 !y
 
-set_option relaxedReassignments true in
 def f4 (x y : Nat) : Nat × Nat := do
-match x with
-| 0 => y := y + 1
-| _ => x := x + y
-return (x, y)
+  let mut (x, y) := (x, y)
+  match x with
+  | 0 => y := y + 1
+  | _ => x := x + y
+  return (x, y)
 
 #eval f4 0 10
 #eval f4 5 10
@@ -135,12 +135,12 @@ rfl
 theorem ex10 (x y : Nat) : f4 (x+1) y = ((x+1)+y, y) :=
 rfl
 
-set_option relaxedReassignments true in
 def f5 (x y : Nat) : Nat × Nat := do
-match x with
-| 0   => y := y + 1
-| z+1 => dbgTrace! "z: {z}"; x := x + y
-return (x, y)
+  let mut (x, y) := (x, y)
+  match x with
+  | 0   => y := y + 1
+  | z+1 => dbgTrace! "z: {z}"; x := x + y
+  return (x, y)
 
 #eval f5 5 6
 
@@ -148,11 +148,11 @@ theorem ex11 (x y : Nat) : f5 (x+1) y = ((x+1)+y, y) :=
 rfl
 
 def f6 (x : Nat) : Nat := do
-let x := x
-if x > 10 then
-  return 0
-x := x + 1
-return x
+  let mut x := x
+  if x > 10 then
+    return 0
+  x := x + 1
+  return x
 
 theorem ex12 : f6 11 = 0 :=
 rfl
diff --git a/tests/lean/run/doNotation3.lean b/tests/lean/run/doNotation3.lean
index 300c6aa49d..58ec09e97e 100644
--- a/tests/lean/run/doNotation3.lean
+++ b/tests/lean/run/doNotation3.lean
@@ -33,7 +33,7 @@ let rec loop (i : Nat) (h : i ≤ as.size) (b : β) : m β := do
 loop as.size (Nat.leRefl _) b
 
 def f (x : Nat) (ref : IO.Ref Nat) : IO Nat := do
-let x := x
+let mut x := x
 if x == 0 then
   x ← ref.get
 IO.println x
@@ -43,12 +43,12 @@ def fTest : IO Unit := do
 unless (← f 0 (← IO.mkRef 10)) == 11 do throw $ IO.userError "unexpected"
 unless (← f 1 (← IO.mkRef 10)) == 2 do throw $ IO.userError "unexpected"
 
-set_option relaxedReassignments true in
 def g (x y : Nat) (ref : IO.Ref (Nat × Nat)) : IO (Nat × Nat) := do
-if x == 0 then
-  (x, y) ← ref.get
-IO.println ("x: " ++ toString x ++ ", y: " ++ toString y)
-return (x, y)
+  let mut (x, y) := (x, y)
+  if x == 0 then
+    (x, y) ← ref.get
+  IO.println ("x: " ++ toString x ++ ", y: " ++ toString y)
+  return (x, y)
 
 def gTest : IO Unit := do
 unless (← g 2 1 (← IO.mkRef (10, 20))) == (2, 1)   do throw $ IO.userError "unexpected"
@@ -59,12 +59,12 @@ return ()
 
 macro "ret!" x:term : doElem => `(return $x)
 
-set_option relaxedReassignments true in
 def f1 (x : Nat) : Nat := do
-if x == 0 then
-  ret! 100
-x := x + 1
-ret! x
+  let mut x := x
+  if x == 0 then
+    ret! 100
+  x := x + 1
+  ret! x
 
 theorem ex1 : f1 0 = 100 := rfl
 theorem ex2 : f1 1 = 2 := rfl
@@ -75,10 +75,10 @@ syntax "inc!" ident : doElem
 macro_rules
 | `(doElem| inc! $x) => `(doElem| $x:ident := $x + 1)
 
-set_option relaxedReassignments true in
 def f2 (x : Nat) : Nat := do
-inc! x
-ret! x
+  let mut x := x
+  inc! x
+  ret! x
 
 theorem ex4 : f2 0 = 1 := rfl
 theorem ex5 : f2 3 = 4 := rfl
diff --git a/tests/lean/run/doNotation4.lean b/tests/lean/run/doNotation4.lean
index a1e499308c..4987ebc142 100644
--- a/tests/lean/run/doNotation4.lean
+++ b/tests/lean/run/doNotation4.lean
@@ -1,5 +1,3 @@
-
-
 abbrev M := StateRefT Nat IO
 
 def testM {α} [ToString α] [BEq α] (init : Nat) (expected : α) (x : M α): IO Unit := do
@@ -52,7 +50,7 @@ catch
 #eval testM 0 2000 $ f3 10
 
 def f4 (xs : List Nat) : M Nat := do
-let y := 0
+let mut y := 0
 for x in xs do
   IO.println s!"x: {x}"
   try
@@ -67,7 +65,7 @@ get
 #eval testM 40 19 $ f4 [1, 2, 3, 4, 5, 6]
 
 def f5 (xs : List Nat) : M Nat := do
-let y := 0
+let mut y := 0
 for x in xs do
   IO.println s!"x: {x}"
   try
diff --git a/tests/lean/run/doNotation6.lean b/tests/lean/run/doNotation6.lean
index e1df055bdc..55453cabcc 100644
--- a/tests/lean/run/doNotation6.lean
+++ b/tests/lean/run/doNotation6.lean
@@ -1,5 +1,3 @@
-
-
 abbrev M := StateRefT Nat IO
 
 def testM {α} [ToString α] [BEq α] (init : Nat) (expected : α) (x : M α): IO Unit := do
@@ -22,7 +20,7 @@ let v ←
     return 1
 
 def f2 (xs : List Nat) : M Nat := do
-let sum := 0
+let mut sum := 0
 for x in xs do
   try
     dec x
@@ -39,7 +37,7 @@ return sum
 #eval testM 1 1 $ f2 [1, 100, 200, 300]
 
 def f3 (xs : List Nat) : M Nat := do
-let sum := 0
+let mut sum := 0
 for x in xs do
   try
     dec x
@@ -56,7 +54,7 @@ return sum
 #eval testM 1 1 $ f3 [1, 100, 200, 300]
 
 def f4 (xs : Array Nat) : IO Nat := do
-let sum := 0
+let mut sum := 0
 for x in xs do
   sum := sum + x
   IO.println x
@@ -65,7 +63,7 @@ return sum
 #eval f4 #[1, 2, 3]
 
 def f5 (xs : Array Nat) : IO Nat := do
-let sum := 0
+let mut sum := 0
 for x in xs[1 : xs.size - 1] do
   sum := sum + x
   IO.println x
diff --git a/tests/lean/run/forInPArray.lean b/tests/lean/run/forInPArray.lean
index 19b83dba06..17e8a1078b 100644
--- a/tests/lean/run/forInPArray.lean
+++ b/tests/lean/run/forInPArray.lean
@@ -6,7 +6,7 @@ unless (← x) == (← expected) do
   throw $ IO.userError "unexpected result"
 
 def f1 (xs : Std.PArray Nat) (top : Nat) : IO Nat := do
-let sum := 0
+let mut sum := 0
 for x in xs do
   if x % 2 == 0 then
     IO.println s!"x: {x}"
@@ -21,7 +21,7 @@ return sum
 #eval check (f1 [1, 2, 3, 4, 5, 10, 20].toPersistentArray 10) (pure 16)
 
 def f2 (xs : Std.PArray Nat) (top : Nat) : IO Nat := do
-let sum := 0
+let mut sum := 0
 for x in xs do
   if x % 2 == 0 then
     IO.println s!"x: {x}"
diff --git a/tests/lean/run/stxMacro.lean b/tests/lean/run/stxMacro.lean
index 32522daa6a..6b8259ac7c 100644
--- a/tests/lean/run/stxMacro.lean
+++ b/tests/lean/run/stxMacro.lean
@@ -1,5 +1,3 @@
-
-
 -- Macro for the `syntax` category
 macro "many " x:stx : stx => `(stx| ($x)*)
 
@@ -7,7 +5,7 @@ syntax "sum! " (many term:max) : term
 
 macro_rules
 | `(sum! $xs*) => do
-  let r ← `(0)
+  let mut r ← `(0)
   for x in xs do
     r ← `($r + $x)
   return r