diff --git a/src/Lean/Compiler/IR/EmitC.lean b/src/Lean/Compiler/IR/EmitC.lean
index bf6489bdd4..3fa7fab2c5 100644
--- a/src/Lean/Compiler/IR/EmitC.lean
+++ b/src/Lean/Compiler/IR/EmitC.lean
@@ -497,8 +497,13 @@ def emitNumLit (t : IRType) (v : Nat) : M Unit := do
   else
     if v < UInt32.size then
       emit v
+    else if t == .usize then
+      emit "((size_t)"
+      emit v
+      emit "ULL)"
     else
-      emit v; emit "ULL"
+      emit v
+      emit "ULL"
 
 def emitLit (z : VarId) (t : IRType) (v : LitVal) : M Unit := do
   emitLhs z;
diff --git a/src/Lean/Compiler/IR/ToIR.lean b/src/Lean/Compiler/IR/ToIR.lean
index 6ca8bd730a..be11ebab2b 100644
--- a/src/Lean/Compiler/IR/ToIR.lean
+++ b/src/Lean/Compiler/IR/ToIR.lean
@@ -70,7 +70,7 @@ def lowerLitValue (v : LCNF.LitValue) : LitVal :=
   | .uint8 v => .num (UInt8.toNat v)
   | .uint16 v => .num (UInt16.toNat v)
   | .uint32 v => .num (UInt32.toNat v)
-  | .uint64 v => .num (UInt64.toNat v)
+  | .uint64 v | .usize v => .num (UInt64.toNat v)
 
 -- TODO: This should be cached.
 def lowerEnumToScalarType (name : Name) : M (Option IRType) := do
diff --git a/src/Lean/Compiler/LCNF/Basic.lean b/src/Lean/Compiler/LCNF/Basic.lean
index 2da4e53f03..41d3e4df4a 100644
--- a/src/Lean/Compiler/LCNF/Basic.lean
+++ b/src/Lean/Compiler/LCNF/Basic.lean
@@ -40,7 +40,9 @@ inductive LitValue where
   | uint16 (val : UInt16)
   | uint32 (val : UInt32)
   | uint64 (val : UInt64)
-  -- TODO: add constructors for `Int`, `Float`, `USize` ...
+  -- USize has a maximum size of 64 bits
+  | usize (val : UInt64)
+  -- TODO: add constructors for `Int`, `Float`, ...
   deriving Inhabited, BEq, Hashable
 
 def LitValue.toExpr : LitValue → Expr
@@ -50,6 +52,7 @@ def LitValue.toExpr : LitValue → Expr
   | .uint16 v => .app (.const ``UInt16.ofNat []) (.lit (.natVal (UInt16.toNat v)))
   | .uint32 v => .app (.const ``UInt32.ofNat []) (.lit (.natVal (UInt32.toNat v)))
   | .uint64 v => .app (.const ``UInt64.ofNat []) (.lit (.natVal (UInt64.toNat v)))
+  | .usize v => .app (.const ``USize.ofNat []) (.lit (.natVal (UInt64.toNat v)))
 
 inductive Arg where
   | erased
diff --git a/src/Lean/Compiler/LCNF/ElimDeadBranches.lean b/src/Lean/Compiler/LCNF/ElimDeadBranches.lean
index 227194df39..52acc3f602 100644
--- a/src/Lean/Compiler/LCNF/ElimDeadBranches.lean
+++ b/src/Lean/Compiler/LCNF/ElimDeadBranches.lean
@@ -177,7 +177,7 @@ def ofLCNFLit : LCNF.LitValue → Value
 | .uint8 v => ofNat (UInt8.toNat v)
 | .uint16 v => ofNat (UInt16.toNat v)
 | .uint32 v => ofNat (UInt32.toNat v)
-| .uint64 v => ofNat (UInt64.toNat v)
+| .uint64 v | .usize v => ofNat (UInt64.toNat v)
 
 partial def proj : Value → Nat → Value
 | .ctor _ vs , i => vs.getD i bot
diff --git a/src/Lean/Compiler/LCNF/InferType.lean b/src/Lean/Compiler/LCNF/InferType.lean
index f6b0aad9dd..95ae15423d 100644
--- a/src/Lean/Compiler/LCNF/InferType.lean
+++ b/src/Lean/Compiler/LCNF/InferType.lean
@@ -109,6 +109,7 @@ def inferLitValueType (value : LitValue) : Expr :=
   | .uint16 .. => mkConst ``UInt16
   | .uint32 .. => mkConst ``UInt32
   | .uint64 .. => mkConst ``UInt64
+  | .usize .. => mkConst ``USize
 
 mutual
   partial def inferArgType (arg : Arg) : InferTypeM Expr :=
diff --git a/src/Lean/Compiler/LCNF/PrettyPrinter.lean b/src/Lean/Compiler/LCNF/PrettyPrinter.lean
index 2032787f44..8b64541fbc 100644
--- a/src/Lean/Compiler/LCNF/PrettyPrinter.lean
+++ b/src/Lean/Compiler/LCNF/PrettyPrinter.lean
@@ -58,7 +58,7 @@ def ppArgs (args : Array Arg) : M Format := do
 
 def ppLitValue (lit : LitValue) : M Format := do
   match lit with
-  | .nat v | .uint8 v | .uint16 v | .uint32 v | .uint64 v => return format v
+  | .nat v | .uint8 v | .uint16 v | .uint32 v | .uint64 v | .usize v => return format v
   | .str v => return format (repr v)
 
 def ppLetValue (e : LetValue) : M Format := do
diff --git a/src/Lean/Compiler/LCNF/Simp/ConstantFold.lean b/src/Lean/Compiler/LCNF/Simp/ConstantFold.lean
index 0dcf52b684..bda10a8fd1 100644
--- a/src/Lean/Compiler/LCNF/Simp/ConstantFold.lean
+++ b/src/Lean/Compiler/LCNF/Simp/ConstantFold.lean
@@ -368,6 +368,7 @@ def conversionFolders : List (Name × Folder) := [
   (``UInt16.ofNat, Folder.ofNat (fun v => .uint16 (UInt16.ofNat v))),
   (``UInt32.ofNat, Folder.ofNat (fun v => .uint32 (UInt32.ofNat v))),
   (``UInt64.ofNat, Folder.ofNat (fun v => .uint64 (UInt64.ofNat v))),
+  (``USize.ofNat, Folder.ofNat (fun v => .usize (UInt64.ofNat v))),
 ]
 
 /--