diff --git a/LANGUAGE.md b/LANGUAGE.md
index bc41e75..eaf2798 100644
--- a/LANGUAGE.md
+++ b/LANGUAGE.md
@@ -9,7 +9,7 @@ roadmap and milestone history.
 
 - newline-terminated statements and `#` comments
 - immutable `::` bindings, typed mutable `=` locals/globals, and `_` sinks
-- immutable package globals, mutable runtime globals, function-local mutable locals, and mutable local declarations initialized with `undefined`
+- immutable package globals, mutable runtime globals, function-local mutable locals, and mutable declarations initialized with `undefined`
 - package-level functions, globals, native type declarations, and `Name :: alias T`
 - directory packages with merged declarations
 - file-local relative imports, import aliases, and qualified member access
diff --git a/TODO.md b/TODO.md
index 0c1ba20..670a3fd 100644
--- a/TODO.md
+++ b/TODO.md
@@ -177,7 +177,7 @@
   - future direction: generalize toward Zig-style arbitrary pointer-result casts once casts have a broader result-type story
 
 12. `undefined` as inspired by zig (implemented):
-  - allow mutable local declarations with `undefined`
+  - allow mutable local and global declarations with `undefined`
     - undefined values are assigned a poison value (0xaa...)
     - allows for something like:
       ```
@@ -696,6 +696,7 @@
     and inferred array counts may resolve through the existing inference fixpoint, but
     the final type must be concrete runtime storage
   - emit source-defined mutable globals as writable globals, not constants
+  - allow `undefined` initializers for runtime storage initialized explicitly by a function
   - allow assignment, address-taking, field/index mutation, and pointer passing under
     the same mutability rules as other writable locations
   - keep mutable globals invalid in comptime evaluation; `$global_var` and writes from
diff --git a/compiler/checker/checker.odin b/compiler/checker/checker.odin
index 5364605..fbdbda6 100644
--- a/compiler/checker/checker.odin
+++ b/compiler/checker/checker.odin
@@ -7891,7 +7891,7 @@ build_compound_expr :: proc(
 		id := source.add(
 			checker.diagnostics,
 			expr.span,
-			"'undefined' is only valid as a mutable local declaration initializer",
+			"'undefined' is only valid as a mutable declaration initializer",
 		)
 		return invalid_hir_expr(checker, expr.span, id, expected)
 	case .Enum_Literal:
@@ -13686,7 +13686,20 @@ build_globals :: proc(checker: ^Checker) {
 		          is_runtime_type(checker, checker.global_types[global_index]) {
 			expected = checker.global_types[global_index]
 		}
-		expr := build_expr(checker, global.expr, nil, &dependencies, &calls, expected, global.pkg, global.file)
+		expr := hir.INVALID_EXPR
+		if !global.immutable && is_undefined_expr(checker, global.expr) {
+			expr = add_hir_expr(checker, hir.Expr{
+				kind=.Undefined,
+				span=checker.ast_module.exprs[global.expr].span,
+				type=checker.global_types[global_index],
+				target=hir.INVALID_REF,
+				left=hir.INVALID_EXPR,
+				right=hir.INVALID_EXPR,
+				diagnostic=source.INVALID_DIAGNOSTIC,
+			})
+		} else {
+			expr = build_expr(checker, global.expr, nil, &dependencies, &calls, expected, global.pkg, global.file)
+		}
 		global_type := checker.global_types[global_index]
 		if is_runtime_type(checker, checker.global_types[global_index]) {
 			expr = coerce_expr(checker, expr, checker.global_types[global_index], global.span)
@@ -13713,17 +13726,6 @@ build_globals :: proc(checker: ^Checker) {
 			global_type = constraint_recovery_type(checker, declared)
 			expr = invalid_hir_expr(checker, global.span, diagnostic, global_type)
 		}
-		if !global.immutable && is_undefined_expr(checker, global.expr) {
-			diagnostic = source.add(
-				checker.diagnostics,
-				global.span,
-				"'undefined' is only valid as a mutable local declaration initializer",
-			)
-			if !is_runtime_type(checker, global_type) {
-				global_type = types.I64
-			}
-			expr = invalid_hir_expr(checker, global.span, diagnostic, global_type)
-		}
 		if diagnostic == source.INVALID_DIAGNOSTIC && !is_runtime_type(checker, global_type) {
 			if types.is_comptime_only(global_type, &checker.module.types) ||
 			   types.is_comptime_only(declared, &checker.module.types) {
diff --git a/compiler_tests.odin b/compiler_tests.odin
index bde4c46..5894bbe 100644
--- a/compiler_tests.odin
+++ b/compiler_tests.odin
@@ -9211,6 +9211,7 @@ counter int = 0
 ratio float = 1
 span range = 0..2
 point Point = Point { x = 1 }
+unset Point = undefined
 values [_]mut i32 = [10, 20]
 main func() void {
 	counter = 1
@@ -9239,8 +9240,9 @@ main func() void {
 	found_counter := false
 	found_ratio := false
 	found_span := false
+	found_unset := false
 	found_values := false
-	for global in hir_module.globals {
+	for global, global_index in hir_module.globals {
 		name := symbol.resolve(&symbols, global.name)
 		if name == "counter" {
 			found_counter = global.writable && !global.is_static && types.equal(global.type, types.I32)
@@ -9248,6 +9250,12 @@ main func() void {
 			found_ratio = global.writable && !global.is_static && types.equal(global.type, types.F64)
 		} else if name == "span" {
 			found_span = global.writable && !global.is_static && types.is_range(global.type, &hir_module.types)
+		} else if name == "unset" {
+			found_unset = global.writable &&
+			              !global.is_static &&
+			              hir_module.exprs[global.expr].kind == .Undefined &&
+			              len(ir_module.globals[global_index].initializer) > 0 &&
+			              ir_module.globals[global_index].initializer[0].op == .Poison
 		} else if name == "values" {
 			item, ok := types.node(&hir_module.types, global.type)
 			found_values = global.writable && !global.is_static && ok && item.kind == .Array && item.count == 2
@@ -9256,6 +9264,7 @@ main func() void {
 	testing.expect(t, found_counter)
 	testing.expect(t, found_ratio)
 	testing.expect(t, found_span)
+	testing.expect(t, found_unset)
 	testing.expect(t, found_values)
 	testing.expect(t, strings.contains(llvm_text, "internal global"))
 	testing.expect(t, !strings.contains(llvm_text, "internal constant i32 0"))
@@ -9267,7 +9276,6 @@ mutable_global_diagnostics_and_no_shadowing :: proc(t: ^testing.T) {
 OtherID :: distinct i32
 ID i32 = 0
 counter = 0
-bad i32 = undefined
 runtime i32 = 1
 immutable :: 1
 foo func(foo i32) void {}
@@ -9304,7 +9312,6 @@ main func() void {
 	defer hir.destroy_module(&hir_module)
 
 	missing_type := false
-	undefined_global := false
 	not_comptime := false
 	immutable_write := false
 	global_type_shadow := false
@@ -9317,7 +9324,6 @@ main func() void {
 	value_label_shadow := false
 	for diagnostic in diagnostics.items {
 		missing_type = missing_type || strings.contains(diagnostic.message, "mutable global 'counter' requires a type annotation")
-		undefined_global = undefined_global || strings.contains(diagnostic.message, "'undefined' is only valid as a mutable local declaration initializer")
 		not_comptime = not_comptime || strings.contains(diagnostic.message, "global 'runtime' is not comptime-known")
 		immutable_write = immutable_write || strings.contains(diagnostic.message, "assignment target is not writable")
 		global_type_shadow = global_type_shadow || strings.contains(diagnostic.message, "global 'ID' shadows visible type")
@@ -9330,7 +9336,6 @@ main func() void {
 		value_label_shadow = value_label_shadow || strings.contains(diagnostic.message, "local 'value_label' shadows visible label")
 	}
 	testing.expect(t, missing_type)
-	testing.expect(t, undefined_global)
 	testing.expect(t, not_comptime)
 	testing.expect(t, immutable_write)
 	testing.expect(t, global_type_shadow)
@@ -11914,7 +11919,7 @@ main func() void {
 	for diagnostic in diagnostics.items {
 		immutable_count += 1 if strings.contains(diagnostic.message, "'undefined' requires a mutable local declaration") else 0
 		found_unresolved = found_unresolved || strings.contains(diagnostic.message, "could not infer a concrete type for local 'unresolved'")
-		found_assignment = found_assignment || strings.contains(diagnostic.message, "'undefined' is only valid as a mutable local declaration initializer")
+		found_assignment = found_assignment || strings.contains(diagnostic.message, "'undefined' is only valid as a mutable declaration initializer")
 		found_incompatible = found_incompatible || strings.contains(diagnostic.message, "cannot implicitly convert f64 to i8")
 	}