diff --git a/lib/std/heap.zig b/lib/std/heap.zig
index f28f5c4b25..f05378c215 100644
--- a/lib/std/heap.zig
+++ b/lib/std/heap.zig
@@ -352,7 +352,7 @@ const WasmPageAllocator = struct {
             return idx + extendedOffset();
         }
 
-        const prev_page_count = @wasmMemoryGrow(@intCast(u32, page_count));
+        const prev_page_count = @wasmMemoryGrow(0, @intCast(u32, page_count));
         if (prev_page_count <= 0) {
             return error.OutOfMemory;
         }
diff --git a/src/all_types.hpp b/src/all_types.hpp
index eaf7203df8..9413ea73a4 100644
--- a/src/all_types.hpp
+++ b/src/all_types.hpp
@@ -3737,21 +3737,27 @@ struct IrInstGenMemcpy {
 
 struct IrInstSrcWasmMemorySize {
     IrInstSrc base;
+
+    IrInstSrc *index;
 };
 
 struct IrInstGenWasmMemorySize {
     IrInstGen base;
+
+    IrInstGen *index;
 };
 
 struct IrInstSrcWasmMemoryGrow {
     IrInstSrc base;
 
+    IrInstSrc *index;
     IrInstSrc *delta;
 };
 
 struct IrInstGenWasmMemoryGrow {
     IrInstGen base;
 
+    IrInstGen *index;
     IrInstGen *delta;
 };
 
diff --git a/src/codegen.cpp b/src/codegen.cpp
index 765d57d7db..f945dd6545 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -5620,26 +5620,16 @@ static LLVMValueRef ir_render_memcpy(CodeGen *g, IrExecutableGen *executable, Ir
 }
 
 static LLVMValueRef ir_render_wasm_memory_size(CodeGen *g, IrExecutableGen *executable, IrInstGenWasmMemorySize *instruction) {
-    // When Wasm lands multi-memory support, we can relax this to permit the user specify
-    // memory index to inquire about. For now, we pass in the recommended default of index 0.
-    //
-    // More info:
-    // https://github.com/sunfishcode/wasm-reference-manual/blob/master/WebAssembly.md#current-linear-memory-size
     // TODO adjust for wasm64
-    LLVMValueRef zero = LLVMConstNull(g->builtin_types.entry_i32->llvm_type);
-    LLVMValueRef val = LLVMBuildCall(g->builder, gen_wasm_memory_size(g), &zero, 1, "");
+    LLVMValueRef param = ir_llvm_value(g, instruction->index);
+    LLVMValueRef val = LLVMBuildCall(g->builder, gen_wasm_memory_size(g), &param, 1, "");
     return val;
 }
 
 static LLVMValueRef ir_render_wasm_memory_grow(CodeGen *g, IrExecutableGen *executable, IrInstGenWasmMemoryGrow *instruction) {
-    // When Wasm lands multi-memory support, we can relax this to permit the user specify
-    // memory index to inquire about. For now, we pass in the recommended default of index 0.
-    //
-    // More info:
-    // https://github.com/sunfishcode/wasm-reference-manual/blob/master/WebAssembly.md#grow-linear-memory-size
     // TODO adjust for wasm64
     LLVMValueRef params[] = {
-        LLVMConstNull(g->builtin_types.entry_i32->llvm_type),
+        ir_llvm_value(g, instruction->index),
         ir_llvm_value(g, instruction->delta),
     };
     LLVMValueRef val = LLVMBuildCall(g->builder, gen_wasm_memory_grow(g), params, 2, "");
@@ -8722,8 +8712,8 @@ static void define_builtin_fns(CodeGen *g) {
     create_builtin_fn(g, BuiltinFnIdAs, "as", 2);
     create_builtin_fn(g, BuiltinFnIdCall, "call", 3);
     create_builtin_fn(g, BuiltinFnIdBitSizeof, "bitSizeOf", 1);
-    create_builtin_fn(g, BuiltinFnIdWasmMemorySize, "wasmMemorySize", 0);
-    create_builtin_fn(g, BuiltinFnIdWasmMemoryGrow, "wasmMemoryGrow", 1);
+    create_builtin_fn(g, BuiltinFnIdWasmMemorySize, "wasmMemorySize", 1);
+    create_builtin_fn(g, BuiltinFnIdWasmMemoryGrow, "wasmMemoryGrow", 2);
 }
 
 static const char *bool_to_str(bool b) {
diff --git a/src/ir.cpp b/src/ir.cpp
index 3edb61556e..aaa9599a1f 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -4985,35 +4985,45 @@ static IrInstGen *ir_build_vector_extract_elem(IrAnalyze *ira, IrInst *source_in
     return &instruction->base;
 }
 
-static IrInstSrc *ir_build_wasm_memory_size_src(IrBuilderSrc *irb, Scope *scope, AstNode *source_node) {
+static IrInstSrc *ir_build_wasm_memory_size_src(IrBuilderSrc *irb, Scope *scope, AstNode *source_node, IrInstSrc *index) {
     IrInstSrcWasmMemorySize *instruction = ir_build_instruction<IrInstSrcWasmMemorySize>(irb, scope, source_node);
+    instruction->index = index;
+
+    ir_ref_instruction(index, irb->current_basic_block);
 
     return &instruction->base;
 }
 
-static IrInstGen *ir_build_wasm_memory_size_gen(IrAnalyze *ira, IrInst *source_instr) {
+static IrInstGen *ir_build_wasm_memory_size_gen(IrAnalyze *ira, IrInst *source_instr, IrInstGen *index) {
     IrInstGenWasmMemorySize *instruction = ir_build_inst_gen<IrInstGenWasmMemorySize>(&ira->new_irb,
             source_instr->scope, source_instr->source_node);
     instruction->base.value->type = ira->codegen->builtin_types.entry_i32;
+    instruction->index = index;
+
+    ir_ref_inst_gen(index);
 
     return &instruction->base;
 }
 
-static IrInstSrc *ir_build_wasm_memory_grow_src(IrBuilderSrc *irb, Scope *scope, AstNode *source_node, IrInstSrc *delta) {
+static IrInstSrc *ir_build_wasm_memory_grow_src(IrBuilderSrc *irb, Scope *scope, AstNode *source_node, IrInstSrc *index, IrInstSrc *delta) {
     IrInstSrcWasmMemoryGrow *instruction = ir_build_instruction<IrInstSrcWasmMemoryGrow>(irb, scope, source_node);
+    instruction->index = index;
     instruction->delta = delta;
 
+    ir_ref_instruction(index, irb->current_basic_block);
     ir_ref_instruction(delta, irb->current_basic_block);
 
     return &instruction->base;
 }
 
-static IrInstGen *ir_build_wasm_memory_grow_gen(IrAnalyze *ira, IrInst *source_instr, IrInstGen *delta) {
+static IrInstGen *ir_build_wasm_memory_grow_gen(IrAnalyze *ira, IrInst *source_instr, IrInstGen *index, IrInstGen *delta) {
     IrInstGenWasmMemoryGrow *instruction = ir_build_inst_gen<IrInstGenWasmMemoryGrow>(&ira->new_irb,
             source_instr->scope, source_instr->source_node);
     instruction->base.value->type = ira->codegen->builtin_types.entry_i32;
+    instruction->index = index;
     instruction->delta = delta;
 
+    ir_ref_inst_gen(index);
     ir_ref_inst_gen(delta);
 
     return &instruction->base;
@@ -6815,7 +6825,12 @@ static IrInstSrc *ir_gen_builtin_fn_call(IrBuilderSrc *irb, Scope *scope, AstNod
             }
         case BuiltinFnIdWasmMemorySize:
             {
-                IrInstSrc *ir_wasm_memory_size = ir_build_wasm_memory_size_src(irb, scope, node);
+                AstNode *arg0_node = node->data.fn_call_expr.params.at(0);
+                IrInstSrc *arg0_value = ir_gen_node(irb, arg0_node, scope);
+                if (arg0_value == irb->codegen->invalid_inst_src)
+                    return arg0_value;
+
+                IrInstSrc *ir_wasm_memory_size = ir_build_wasm_memory_size_src(irb, scope, node, arg0_value);
                 return ir_lval_wrap(irb, scope, ir_wasm_memory_size, lval, result_loc);
             }
         case BuiltinFnIdWasmMemoryGrow:
@@ -6825,7 +6840,12 @@ static IrInstSrc *ir_gen_builtin_fn_call(IrBuilderSrc *irb, Scope *scope, AstNod
                 if (arg0_value == irb->codegen->invalid_inst_src)
                     return arg0_value;
 
-                IrInstSrc *ir_wasm_memory_grow = ir_build_wasm_memory_grow_src(irb, scope, node, arg0_value);
+                AstNode *arg1_node = node->data.fn_call_expr.params.at(1);
+                IrInstSrc *arg1_value = ir_gen_node(irb, arg1_node, scope);
+                if (arg1_value == irb->codegen->invalid_inst_src)
+                    return arg1_value;
+
+                IrInstSrc *ir_wasm_memory_grow = ir_build_wasm_memory_grow_src(irb, scope, node, arg0_value, arg1_value);
                 return ir_lval_wrap(irb, scope, ir_wasm_memory_grow, lval, result_loc);
             }
         case BuiltinFnIdField:
@@ -27733,7 +27753,17 @@ static IrInstGen *ir_analyze_instruction_wasm_memory_size(IrAnalyze *ira, IrInst
         return ira->codegen->invalid_inst_gen;
     }
 
-    return ir_build_wasm_memory_size_gen(ira, &instruction->base.base);
+    IrInstGen *index = instruction->index->child;
+    if (type_is_invalid(index->value->type))
+        return ira->codegen->invalid_inst_gen;
+
+    ZigType *i32_type = ira->codegen->builtin_types.entry_i32;
+
+    IrInstGen *casted_index = ir_implicit_cast(ira, index, i32_type);
+    if (type_is_invalid(casted_index->value->type))
+        return ira->codegen->invalid_inst_gen;
+
+    return ir_build_wasm_memory_size_gen(ira, &instruction->base.base, casted_index);
 }
 
 static IrInstGen *ir_analyze_instruction_wasm_memory_grow(IrAnalyze *ira, IrInstSrcWasmMemoryGrow *instruction) {
@@ -27744,17 +27774,25 @@ static IrInstGen *ir_analyze_instruction_wasm_memory_grow(IrAnalyze *ira, IrInst
         return ira->codegen->invalid_inst_gen;
     }
 
-    IrInstGen *delta = instruction->delta->child;
-    if (type_is_invalid(delta->value->type))
+    IrInstGen *index = instruction->index->child;
+    if (type_is_invalid(index->value->type))
         return ira->codegen->invalid_inst_gen;
 
     ZigType *i32_type = ira->codegen->builtin_types.entry_i32;
 
+    IrInstGen *casted_index = ir_implicit_cast(ira, index, i32_type);
+    if (type_is_invalid(casted_index->value->type))
+        return ira->codegen->invalid_inst_gen;
+
+    IrInstGen *delta = instruction->delta->child;
+    if (type_is_invalid(delta->value->type))
+        return ira->codegen->invalid_inst_gen;
+
     IrInstGen *casted_delta = ir_implicit_cast(ira, delta, i32_type);
     if (type_is_invalid(casted_delta->value->type))
         return ira->codegen->invalid_inst_gen;
 
-    return ir_build_wasm_memory_grow_gen(ira, &instruction->base.base, casted_delta);
+    return ir_build_wasm_memory_grow_gen(ira, &instruction->base.base, casted_index, casted_delta);
 }
 
 static IrInstGen *ir_analyze_instruction_breakpoint(IrAnalyze *ira, IrInstSrcBreakpoint *instruction) {
diff --git a/src/ir_print.cpp b/src/ir_print.cpp
index e287ca74fc..c826d76e03 100644
--- a/src/ir_print.cpp
+++ b/src/ir_print.cpp
@@ -1717,21 +1717,29 @@ static void ir_print_bool_not(IrPrintGen *irp, IrInstGenBoolNot *instruction) {
 }
 
 static void ir_print_wasm_memory_size(IrPrintSrc *irp, IrInstSrcWasmMemorySize *instruction) {
-    fprintf(irp->f, "@wasmMemorySize()");
+    fprintf(irp->f, "@wasmMemorySize(");
+    ir_print_other_inst_src(irp, instruction->index);
+    fprintf(irp->f, ")");
 }
 
 static void ir_print_wasm_memory_size(IrPrintGen *irp, IrInstGenWasmMemorySize *instruction) {
-    fprintf(irp->f, "@wasmMemorySize()");
+    fprintf(irp->f, "@wasmMemorySize(");
+    ir_print_other_inst_gen(irp, instruction->index);
+    fprintf(irp->f, ")");
 }
 
 static void ir_print_wasm_memory_grow(IrPrintSrc *irp, IrInstSrcWasmMemoryGrow *instruction) {
     fprintf(irp->f, "@wasmMemoryGrow(");
+    ir_print_other_inst_src(irp, instruction->index);
+    fprintf(irp->f, ", ");
     ir_print_other_inst_src(irp, instruction->delta);
     fprintf(irp->f, ")");
 }
 
 static void ir_print_wasm_memory_grow(IrPrintGen *irp, IrInstGenWasmMemoryGrow *instruction) {
     fprintf(irp->f, "@wasmMemoryGrow(");
+    ir_print_other_inst_gen(irp, instruction->index);
+    fprintf(irp->f, ", ");
     ir_print_other_inst_gen(irp, instruction->delta);
     fprintf(irp->f, ")");
 }
diff --git a/test/compile_errors.zig b/test/compile_errors.zig
index 77438e5bb0..278d66ab9f 100644
--- a/test/compile_errors.zig
+++ b/test/compile_errors.zig
@@ -7507,7 +7507,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
 
     cases.add("wasmMemorySize is a compile error in non-Wasm targets",
         \\export fn foo() void {
-        \\    _ = @wasmMemorySize();
+        \\    _ = @wasmMemorySize(0);
         \\    return;
         \\}
     , &[_][]const u8{
@@ -7516,7 +7516,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
 
     cases.add("wasmMemoryGrow is a compile error in non-Wasm targets",
         \\export fn foo() void {
-        \\    _ = @wasmMemoryGrow(1);
+        \\    _ = @wasmMemoryGrow(0, 1);
         \\    return;
         \\}
     , &[_][]const u8{