diff --git a/src/Air.zig b/src/Air.zig
index 11e7045da3..0a8771f6ac 100644
--- a/src/Air.zig
+++ b/src/Air.zig
@@ -583,6 +583,10 @@ pub const Inst = struct {
         /// Uses the `ty_pl` field.
         field_parent_ptr,
 
+        /// Implements @wasmMemorySize builtin.
+        /// Uses the `ty_pl` field, payload is `WasmMemoryIndex`.
+        wasm_memory_size,
+
         pub fn fromCmpOp(op: std.math.CompareOperator) Tag {
             return switch (op) {
                 .lt => .cmp_lt,
@@ -717,6 +721,12 @@ pub const FieldParentPtr = struct {
     field_index: u32,
 };
 
+/// Wasm's memory instructions require a comptime-known index
+/// which represents the memory it operates on.
+pub const WasmMemoryIndex = struct {
+    index: u32,
+};
+
 /// Trailing:
 /// 0. `Inst.Ref` for every outputs_len
 /// 1. `Inst.Ref` for every inputs_len
@@ -877,6 +887,7 @@ pub fn typeOfIndex(air: Air, inst: Air.Inst.Index) Type {
         .aggregate_init,
         .union_init,
         .field_parent_ptr,
+        .wasm_memory_size,
         => return air.getRefType(datas[inst].ty_pl.ty),
 
         .not,
diff --git a/src/AstGen.zig b/src/AstGen.zig
index 2fee02751c..a82d9dc360 100644
--- a/src/AstGen.zig
+++ b/src/AstGen.zig
@@ -7140,7 +7140,7 @@ fn builtinCall(
         // zig fmt: on
 
         .wasm_memory_size => {
-            const operand = try expr(gz, scope, .{ .ty = .u32_type }, params[0]);
+            const operand = try comptimeExpr(gz, scope, .{ .ty = .u32_type }, params[0]);
             const result = try gz.addExtendedPayload(.wasm_memory_size, Zir.Inst.UnNode{
                 .node = gz.nodeIndexToRelative(node),
                 .operand = operand,
diff --git a/src/Liveness.zig b/src/Liveness.zig
index d47c7086ce..e90a9ab10e 100644
--- a/src/Liveness.zig
+++ b/src/Liveness.zig
@@ -318,6 +318,7 @@ fn analyzeInst(
         .fence,
         .ret_addr,
         .frame_addr,
+        .wasm_memory_size,
         => return trackOperands(a, new_set, inst, main_tomb, .{ .none, .none, .none }),
 
         .not,
diff --git a/src/Sema.zig b/src/Sema.zig
index fa25e1e403..3dbbdaeb27 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -14034,7 +14034,17 @@ fn zirWasmMemorySize(
 ) CompileError!Air.Inst.Ref {
     const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
     const src: LazySrcLoc = .{ .node_offset = extra.node };
-    return sema.fail(block, src, "TODO: implement Sema.zirWasmMemorySize", .{});
+
+    const operand = try sema.resolveInt(block, src, extra.operand, Type.u32);
+    const index = @intCast(u32, operand);
+    try sema.requireRuntimeBlock(block, src);
+    return block.addInst(.{
+        .tag = .wasm_memory_size,
+        .data = .{ .ty_pl = .{
+            .ty = try sema.addType(Type.u32),
+            .payload = try sema.addExtra(Air.WasmMemoryIndex{ .index = index }),
+        } },
+    });
 }
 
 fn zirWasmMemoryGrow(
diff --git a/src/arch/wasm/CodeGen.zig b/src/arch/wasm/CodeGen.zig
index 4f19aeca6b..0ea0418d7f 100644
--- a/src/arch/wasm/CodeGen.zig
+++ b/src/arch/wasm/CodeGen.zig
@@ -1671,6 +1671,8 @@ fn genInst(self: *Self, inst: Air.Inst.Index) !WValue {
         .wrap_errunion_payload => self.airWrapErrUnionPayload(inst),
         .wrap_errunion_err => self.airWrapErrUnionErr(inst),
 
+        .wasm_memory_size => self.airWasmMemorySize(inst),
+
         .add_sat,
         .sub_sat,
         .mul_sat,
@@ -3426,6 +3428,16 @@ fn airPrefetch(self: *Self, inst: Air.Inst.Index) InnerError!WValue {
     return WValue{ .none = {} };
 }
 
+fn airWasmMemorySize(self: *Self, inst: Air.Inst.Index) !WValue {
+    const ty_pl = self.air.instructions.items(.data)[inst].ty_pl;
+    const extra = self.air.extraData(Air.WasmMemoryIndex, ty_pl.payload).data;
+
+    const result = try self.allocLocal(Type.usize);
+    try self.addLabel(.memory_size, extra.index);
+    try self.addLabel(.local_set, result.local);
+    return result;
+}
+
 fn cmpOptionals(self: *Self, lhs: WValue, rhs: WValue, operand_ty: Type, op: std.math.CompareOperator) InnerError!WValue {
     assert(operand_ty.hasRuntimeBits());
     assert(op == .eq or op == .neq);
diff --git a/src/arch/wasm/Emit.zig b/src/arch/wasm/Emit.zig
index d787c46774..4432e19638 100644
--- a/src/arch/wasm/Emit.zig
+++ b/src/arch/wasm/Emit.zig
@@ -89,10 +89,10 @@ pub fn emitMir(emit: *Emit) InnerError!void {
             .local_set => try emit.emitLabel(tag, inst),
             .local_tee => try emit.emitLabel(tag, inst),
             .memory_grow => try emit.emitLabel(tag, inst),
+            .memory_size => try emit.emitLabel(tag, inst),
 
             // no-ops
             .end => try emit.emitTag(tag),
-            .memory_size => try emit.emitTag(tag),
             .@"return" => try emit.emitTag(tag),
             .@"unreachable" => try emit.emitTag(tag),
 
diff --git a/src/arch/wasm/Mir.zig b/src/arch/wasm/Mir.zig
index ed0867e583..04edb09dca 100644
--- a/src/arch/wasm/Mir.zig
+++ b/src/arch/wasm/Mir.zig
@@ -226,9 +226,9 @@ pub const Inst = struct {
         i64_store32 = 0x3E,
         /// Returns the memory size in amount of pages.
         ///
-        /// Uses `nop`
+        /// Uses `label`
         memory_size = 0x3F,
-        /// Increases the memory at by given number of pages.
+        /// Increases the memory by given number of pages.
         ///
         /// Uses `label`
         memory_grow = 0x40,
diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index 252d449fc2..a22d8e01b0 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -2314,6 +2314,8 @@ pub const FuncGen = struct {
                 .wrap_errunion_payload => try self.airWrapErrUnionPayload(inst),
                 .wrap_errunion_err     => try self.airWrapErrUnionErr(inst),
 
+                .wasm_memory_size => try self.airWasmMemorySize(inst),
+
                 .constant => unreachable,
                 .const_ty => unreachable,
                 .unreach  => self.airUnreach(inst),
@@ -3474,6 +3476,11 @@ pub const FuncGen = struct {
         return partial;
     }
 
+    fn airWasmMemorySize(self: *FuncGen, inst: Air.Inst.Index) !?*const llvm.Value {
+        _ = inst;
+        return self.todo("`@wasmMemorySize()`", .{});
+    }
+
     fn airMin(self: *FuncGen, inst: Air.Inst.Index) !?*const llvm.Value {
         if (self.liveness.isUnused(inst)) return null;
 
diff --git a/src/print_air.zig b/src/print_air.zig
index ae6959e0fc..4129fc5c43 100644
--- a/src/print_air.zig
+++ b/src/print_air.zig
@@ -250,6 +250,7 @@ const Writer = struct {
             .memcpy => try w.writeMemcpy(s, inst),
             .memset => try w.writeMemset(s, inst),
             .field_parent_ptr => try w.writeFieldParentPtr(s, inst),
+            .wasm_memory_size => try w.writeWasmMemorySize(s, inst),
 
             .add_with_overflow,
             .sub_with_overflow,
@@ -623,6 +624,13 @@ const Writer = struct {
         try s.writeAll("}");
     }
 
+    fn writeWasmMemorySize(w: *Writer, s: anytype, inst: Air.Inst.Index) @TypeOf(s).Error!void {
+        const pl_op = w.air.instructions.items(.data)[inst].ty_pl;
+        const extra = w.air.extraData(Air.WasmMemoryIndex, pl_op.payload).data;
+
+        try s.print(", {d}", .{extra.index});
+    }
+
     fn writeOperand(
         w: *Writer,
         s: anytype,