diff --git a/automation/v4-docs/gui.txt b/automation/v4-docs/gui.txt
index 1ec617805..f913e7040 100644
--- a/automation/v4-docs/gui.txt
+++ b/automation/v4-docs/gui.txt
@@ -55,3 +55,12 @@ Returns: 0 values
 
 ---
 
+Determining whether there are unsaved changes
+
+function aegisub.gui.is_modified()
+
+Returns: 1 boolean
+  1. Whether the current file has unsaved changes.
+
+---
+
diff --git a/src/auto4_lua.cpp b/src/auto4_lua.cpp
index bba6e6aeb..cb51c8e49 100644
--- a/src/auto4_lua.cpp
+++ b/src/auto4_lua.cpp
@@ -319,6 +319,12 @@ namespace {
 		return 0;
 	}
 
+	int lua_is_modified(lua_State *L)
+	{
+		push_value(L, get_context(L)->subsController->IsModified());
+		return 1;
+	}
+
 	int project_properties(lua_State *L)
 	{
 		const agi::Context *c = get_context(L);
@@ -523,11 +529,12 @@ namespace {
 		set_field<project_properties>(L, "project_properties");
 		set_field<lua_get_audio_selection>(L, "get_audio_selection");
 		set_field<lua_set_status_text>(L, "set_status_text");
-		lua_createtable(L, 0, 4);
+		lua_createtable(L, 0, 5);
 		set_field<lua_get_text_cursor>(L, "get_cursor");
 		set_field<lua_set_text_cursor>(L, "set_cursor");
 		set_field<lua_get_text_selection>(L, "get_selection");
 		set_field<lua_set_text_selection>(L, "set_selection");
+		set_field<lua_is_modified>(L, "is_modified");
 		lua_setfield(L, -2, "gui");
 
 		// store aegisub table to globals