diff --git a/src/check_builtin.cpp b/src/check_builtin.cpp
index b2afa3d0c..a74082fb2 100644
--- a/src/check_builtin.cpp
+++ b/src/check_builtin.cpp
@@ -1756,6 +1756,17 @@ gb_internal bool check_builtin_procedure_directive(CheckerContext *c, Operand *o
 		operand->mode = Addressing_Constant;
 		operand->value = exact_value_bool(is_defined);
 
+		// If the arg is a selector expression we don't add it, `-define` only allows identifiers.
+		if (arg->kind == Ast_Ident) {
+			Defineable defineable    = {};
+			defineable.name          = arg->Ident.token.string;
+			defineable.default_value = exact_value_bool(false);
+			defineable.pos           = arg->Ident.token.pos;
+
+			MUTEX_GUARD(&c->info->defineables_mutex);
+			array_add(&c->info->defineables, defineable);
+		}
+
 	} else if (name == "config") {
 		if (ce->args.count != 2) {
 			error(call, "'#config' expects 2 argument, got %td", ce->args.count);
@@ -1777,7 +1788,7 @@ gb_internal bool check_builtin_procedure_directive(CheckerContext *c, Operand *o
 		}
 
 		String name = arg->Ident.token.string;
-		
+
 
 		operand->type = def.type;
 		operand->mode = def.mode;
@@ -1794,7 +1805,7 @@ gb_internal bool check_builtin_procedure_directive(CheckerContext *c, Operand *o
 			}
 		}
 
-		Defineable defineable = {};
+		Defineable defineable    = {};
 		defineable.name          = name;
 		defineable.default_value = def.value;
 		defineable.pos           = arg->Ident.token.pos;
diff --git a/src/checker.hpp b/src/checker.hpp
index fc37f5769..9db5757e3 100644
--- a/src/checker.hpp
+++ b/src/checker.hpp
@@ -381,6 +381,7 @@ struct Defineable {
 	ExactValue default_value;
 	TokenPos   pos;
 
+	// These strings are only computed from previous fields when defineables are being shown or exported.
 	String default_value_str;
 	String pos_str;
 };
diff --git a/src/main.cpp b/src/main.cpp
index a3e87fb6a..eac1b7ed9 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -822,9 +822,9 @@ gb_internal bool parse_build_flags(Array<String> args) {
 						}
 				   		case BuildFlag_ShowDefineables: {
 							GB_ASSERT(value.kind == ExactValue_Invalid);
-				   			build_context.show_defineables = true;
-				   			break;
-				   		}
+							build_context.show_defineables = true;
+							break;
+						}
 						case BuildFlag_ExportDefineables: {
 							GB_ASSERT(value.kind == ExactValue_String);
 
@@ -1577,6 +1577,28 @@ gb_internal void timings_export_all(Timings *t, Checker *c, bool timings_are_fin
 	gb_printf("Done.\n");
 }
 
+gb_internal void check_defines(BuildContext *bc, Checker *c) {
+	for (auto const &entry : bc->defined_values) {
+		String name = make_string_c(entry.key);
+		ExactValue value = entry.value;
+		GB_ASSERT(value.kind != ExactValue_Invalid);
+		
+		bool found = false;
+		for_array(i, c->info.defineables) {
+			Defineable *def = &c->info.defineables[i];
+			if (def->name == name) {
+				found = true;
+				break;
+			}
+		}
+
+		if (!found) {
+			warning(nullptr, "given -define:%s is unused in the project", name);
+			error_line("\tSuggestion: use the -show-defineables flag for an overview of the possible defines\n");
+		}
+	}
+}
+
 gb_internal void temp_alloc_defineable_strings(Checker *c) {
 	for_array(i, c->info.defineables) {
 		Defineable *def = &c->info.defineables[i];
@@ -2059,11 +2081,11 @@ gb_internal void print_show_help(String const arg0, String const &command) {
 		print_usage_line(0, "");
 
 		print_usage_line(1, "-show-defineables");
-		print_usage_line(2, "Shows an overview of all the #config usages in the project.");
+		print_usage_line(2, "Shows an overview of all the #config/#defined usages in the project.");
 		print_usage_line(0, "");
 
 		print_usage_line(1, "-export-defineables:<filename>");
-		print_usage_line(2, "Exports an overview of all the #config usages in CSV format to the given file path.");
+		print_usage_line(2, "Exports an overview of all the #config/#defined usages in CSV format to the given file path.");
 		print_usage_line(2, "Example: -export-defineables:defineables.csv");
 		print_usage_line(0, "");
 	}
@@ -3063,6 +3085,7 @@ int main(int arg_count, char const **arg_ptr) {
 	defer (destroy_checker(checker));
 
 	check_parsed_files(checker);
+	check_defines(&build_context, checker);
 	if (any_errors()) {
 		print_all_errors();
 		return 1;