diff --git a/src/scripting/thingdef_data.cpp b/src/scripting/thingdef_data.cpp
index 7cfa53504..f7b84f21b 100644
--- a/src/scripting/thingdef_data.cpp
+++ b/src/scripting/thingdef_data.cpp
@@ -1252,3 +1252,18 @@ DEFINE_ACTION_FUNCTION(FStringStruct, Filter)
 	ACTION_RETURN_STRING(strbin1(*self));
 }
 
+DEFINE_ACTION_FUNCTION(FStringStruct, IndexOf)
+{
+	PARAM_SELF_STRUCT_PROLOGUE(FString);
+	PARAM_STRING(substr);
+	PARAM_INT_DEF(startIndex);
+	ACTION_RETURN_INT(self->IndexOf(substr, startIndex));
+}
+
+DEFINE_ACTION_FUNCTION(FStringStruct, LastIndexOf)
+{
+	PARAM_SELF_STRUCT_PROLOGUE(FString);
+	PARAM_STRING(substr);
+	ACTION_RETURN_INT(self->LastIndexOf(substr));
+}
+
diff --git a/wadsrc/static/zscript/base.txt b/wadsrc/static/zscript/base.txt
index f35e39993..b2fd9c6c8 100644
--- a/wadsrc/static/zscript/base.txt
+++ b/wadsrc/static/zscript/base.txt
@@ -656,6 +656,8 @@ struct StringStruct native
 	native String CharAt(int pos) const;
 	native int CharCodeAt(int pos) const;
 	native String Filter();
+	native int IndexOf(String substr, int startIndex = 0) const;
+	native int LastIndexOf(String substr) const;
 }
 
 class SectorEffect : Thinker native