diff --git a/src/dobjgc.cpp b/src/dobjgc.cpp
index ebf61a698..d52962f11 100644
--- a/src/dobjgc.cpp
+++ b/src/dobjgc.cpp
@@ -307,6 +307,7 @@ static void MarkRoot()
 	DThinker::MarkRoots();
 	FCanvasTextureInfo::Mark();
 	Mark(DACSThinker::ActiveThinker);
+	Mark(level.DefaultSkybox);
 	// Mark dead bodies.
 	for (i = 0; i < BODYQUESIZE; ++i)
 	{
diff --git a/src/g_level.cpp b/src/g_level.cpp
index e01152924..3b0c8fe12 100644
--- a/src/g_level.cpp
+++ b/src/g_level.cpp
@@ -77,6 +77,7 @@
 #include "d_netinf.h"
 #include "v_palette.h"
 #include "menu/menu.h"
+#include "a_sharedglobal.h"
 #include "a_strifeglobal.h"
 #include "r_data/colormaps.h"
 #include "farchive.h"
@@ -1272,6 +1273,7 @@ void G_InitLevelLocals ()
 	NormalLight.ChangeFade (level.fadeto);
 
 	level.DefaultEnvironment = info->DefaultEnvironment;
+	level.DefaultSkybox = NULL;
 }
 
 //==========================================================================
@@ -1435,6 +1437,11 @@ void G_SerializeLevel (FArchive &arc, bool hubLoad)
 	P_SerializeSubsectors(arc);
 	StatusBar->Serialize (arc);
 
+	if (SaveVersion >= 4222)
+	{ // This must be done *after* thinkers are serialized.
+		arc << level.DefaultSkybox;
+	}
+
 	arc << level.total_monsters << level.total_items << level.total_secrets;
 
 	// Does this level have custom translations?
diff --git a/src/g_level.h b/src/g_level.h
index cd8f6c1f7..e7d1cbbd2 100644
--- a/src/g_level.h
+++ b/src/g_level.h
@@ -426,6 +426,8 @@ struct FLevelLocals
 	int			airsupply;
 	int			DefaultEnvironment;		// Default sound environment.
 
+	TObjPtr<class ASkyViewpoint> DefaultSkybox;
+
 	FSectorScrollValues	*Scrolls;		// NULL if no DScrollers in this level
 
 	SBYTE		WallVertLight;			// Light diffs for vert/horiz walls
diff --git a/src/g_shared/a_skies.cpp b/src/g_shared/a_skies.cpp
index 5e894002d..616b87c9e 100644
--- a/src/g_shared/a_skies.cpp
+++ b/src/g_shared/a_skies.cpp
@@ -49,21 +49,9 @@ void ASkyViewpoint::BeginPlay ()
 {
 	Super::BeginPlay ();
 
-	if (tid == 0)
+	if (tid == 0 && level.DefaultSkybox == NULL)
 	{
-		int i;
-
-		for (i = 0; i <numsectors; i++)
-		{
-			if (sectors[i].FloorSkyBox == NULL)
-			{
-				sectors[i].FloorSkyBox = this;
-			}
-			if (sectors[i].CeilingSkyBox == NULL)
-			{
-				sectors[i].CeilingSkyBox = this;
-			}
-		}
+		level.DefaultSkybox = this;
 	}
 }
 
@@ -87,6 +75,10 @@ void ASkyViewpoint::Destroy ()
 			sectors[i].CeilingSkyBox = NULL;
 		}
 	}
+	if (level.DefaultSkybox == this)
+	{
+		level.DefaultSkybox = NULL;
+	}
 	Super::Destroy();
 }
 
diff --git a/src/r_bsp.cpp b/src/r_bsp.cpp
index 9ada2fdec..31dfe9907 100644
--- a/src/r_bsp.cpp
+++ b/src/r_bsp.cpp
@@ -1015,6 +1015,7 @@ void R_Subsector (subsector_t *sub)
 	int          ceilinglightlevel;		// killough 4/11/98
 	bool		 outersubsector;
 	int	fll, cll, position;
+	ASkyViewpoint *skybox;
 
 	// kg3D - fake floor stuff
 	visplane_t *backupfp;
@@ -1082,9 +1083,10 @@ void R_Subsector (subsector_t *sub)
 		basecolormap = frontsector->ColorMap;
 	}
 
+	skybox = frontsector->CeilingSkyBox != NULL ? frontsector->CeilingSkyBox : level.DefaultSkybox;
 	ceilingplane = frontsector->ceilingplane.PointOnSide(viewx, viewy, viewz) > 0 ||
 		frontsector->GetTexture(sector_t::ceiling) == skyflatnum ||
-		(frontsector->CeilingSkyBox != NULL && frontsector->CeilingSkyBox->bAlways) ||
+		(skybox != NULL && skybox->bAlways) ||
 		(frontsector->heightsec && 
 		 !(frontsector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC) &&
 		 frontsector->heightsec->GetTexture(sector_t::floor) == skyflatnum) ?
@@ -1099,7 +1101,7 @@ void R_Subsector (subsector_t *sub)
 					frontsector->GetYScale(sector_t::ceiling),
 					frontsector->GetAngle(sector_t::ceiling),
 					frontsector->sky,
-					frontsector->CeilingSkyBox
+					skybox
 					) : NULL;
 
 	if (fixedlightlev < 0 && frontsector->e && frontsector->e->XFloor.lightlist.Size())
@@ -1121,9 +1123,10 @@ void R_Subsector (subsector_t *sub)
 	// killough 3/7/98: Add (x,y) offsets to flats, add deep water check
 	// killough 3/16/98: add floorlightlevel
 	// killough 10/98: add support for skies transferred from sidedefs
+	skybox = frontsector->FloorSkyBox != NULL ? frontsector->FloorSkyBox : level.DefaultSkybox;
 	floorplane = frontsector->floorplane.PointOnSide(viewx, viewy, viewz) > 0 || // killough 3/7/98
 		frontsector->GetTexture(sector_t::floor) == skyflatnum ||
-		(frontsector->FloorSkyBox != NULL && frontsector->FloorSkyBox->bAlways) ||
+		(skybox != NULL && skybox->bAlways) ||
 		(frontsector->heightsec &&
 		 !(frontsector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC) &&
 		 frontsector->heightsec->GetTexture(sector_t::ceiling) == skyflatnum) ?
@@ -1138,7 +1141,7 @@ void R_Subsector (subsector_t *sub)
 					frontsector->GetYScale(sector_t::floor),
 					frontsector->GetAngle(sector_t::floor),
 					frontsector->sky,
-					frontsector->FloorSkyBox
+					skybox
 					) : NULL;
 
 	// kg3D - fake planes rendering