diff --git a/src/maploader/maploader.cpp b/src/maploader/maploader.cpp
index 3159fa7b78..983cd31a2b 100644
--- a/src/maploader/maploader.cpp
+++ b/src/maploader/maploader.cpp
@@ -3660,6 +3660,22 @@ DEFINE_ACTION_FUNCTION(DLevelPostProcessor, SetVertex)
 	return 0;
 }
 
+DEFINE_ACTION_FUNCTION(DLevelPostProcessor, FlipLine)
+{
+	PARAM_SELF_PROLOGUE(DLevelPostProcessor);
+	PARAM_UINT(lineidx);
+
+	line_t *line = &self->Level->lines[lineidx];
+	vertex_t *v1 = line->v1;
+	vertex_t *v2 = line->v2;
+
+	line->v1 = v2;
+	line->v2 = v1;
+
+	self->loader->ForceNodeBuild = true;
+	return 0;
+}
+
 DEFINE_ACTION_FUNCTION(DLevelPostProcessor, SetLineSectorRef)
 {
 	PARAM_SELF_PROLOGUE(DLevelPostProcessor);
diff --git a/wadsrc/static/zscript/level_postprocessor.zs b/wadsrc/static/zscript/level_postprocessor.zs
index afef058d58..fb2ac6d957 100644
--- a/wadsrc/static/zscript/level_postprocessor.zs
+++ b/wadsrc/static/zscript/level_postprocessor.zs
@@ -47,6 +47,7 @@ class LevelPostProcessor native play
 	protected native void SetThingStringArgument(uint thing, Name value);
 
 	protected native void SetVertex(uint vertex, double x, double y);
+	protected native void FlipLine(uint Line);
 	protected native void SetLineSectorRef(uint line, uint side, uint sector);
 	protected native Actor GetDefaultActor(Name actorclass);