diff --git a/src/gl/scene/gl_sky.cpp b/src/gl/scene/gl_sky.cpp
index 17e864476c..2b3b06eaac 100644
--- a/src/gl/scene/gl_sky.cpp
+++ b/src/gl/scene/gl_sky.cpp
@@ -131,9 +131,10 @@ void GLSkyInfo::init(int sky1, PalEntry FadeColor)
 
 //==========================================================================
 //
-//  Calculate sky texture
+//  Calculate sky texture for ceiling or floor
 //
 //==========================================================================
+
 void GLWall::SkyPlane(sector_t *sector, int plane, bool allowreflect)
 {
 	FPortal *portal = sector->portals[plane];
@@ -178,6 +179,35 @@ void GLWall::SkyPlane(sector_t *sector, int plane, bool allowreflect)
 }
 
 
+//==========================================================================
+//
+//  Calculate sky texture for a line
+//
+//==========================================================================
+
+void GLWall::SkyLine(line_t *line)
+{
+	ASkyViewpoint * skyboxx = line->skybox;
+	GLSkyInfo skyinfo;
+
+	// JUSTHIT is used as an indicator that a skybox is in use.
+	// This is to avoid recursion
+
+	if (!gl_noskyboxes && skyboxx && GLRenderer->mViewActor != skyboxx && !(skyboxx->flags&MF_JUSTHIT))
+	{
+		type = RENDERWALL_SKYBOX;
+		skybox = skyboxx;
+	}
+	else
+	{
+		skyinfo.init(line->frontsector->sky, Colormap.FadeColor);
+		type = RENDERWALL_SKY;
+		sky = UniqueSkies.Get(&skyinfo);
+	}
+	PutWall(0);
+}
+
+
 //==========================================================================
 //
 //  Skies on one sided walls
@@ -191,6 +221,15 @@ void GLWall::SkyNormal(sector_t * fs,vertex_t * v1,vertex_t * v2)
 	zbottom[1]=zceil[1];
 	SkyPlane(fs, sector_t::ceiling, true);
 
+	if (seg->linedef->skybox != NULL)
+	{
+		ztop[0] = zceil[0];
+		ztop[1] = zceil[1];
+		zbottom[0] = zfloor[0];
+		zbottom[1] = zfloor[1];
+		SkyLine(seg->linedef);
+	}
+
 	ztop[0]=zfloor[0];
 	ztop[1]=zfloor[1];
 	zbottom[0]=zbottom[1]=-32768.0f;
diff --git a/src/gl/scene/gl_wall.h b/src/gl/scene/gl_wall.h
index 47ac06c466..b7f8653e9d 100644
--- a/src/gl/scene/gl_wall.h
+++ b/src/gl/scene/gl_wall.h
@@ -171,6 +171,7 @@ private:
 	void FloodPlane(int pass);
 
 	void SkyPlane(sector_t *sector, int plane, bool allowmirror);
+	void SkyLine(line_t *line);
 	void SkyNormal(sector_t * fs,vertex_t * v1,vertex_t * v2);
 	void SkyTop(seg_t * seg,sector_t * fs,sector_t * bs,vertex_t * v1,vertex_t * v2);
 	void SkyBottom(seg_t * seg,sector_t * fs,sector_t * bs,vertex_t * v1,vertex_t * v2);
diff --git a/src/gl/scene/gl_walls.cpp b/src/gl/scene/gl_walls.cpp
index ad2a3aa93f..bb23922954 100644
--- a/src/gl/scene/gl_walls.cpp
+++ b/src/gl/scene/gl_walls.cpp
@@ -1569,13 +1569,16 @@ void GLWall::Process(seg_t *seg, sector_t * frontsector, sector_t * backsector)
 		// sector's sky
 		SkyNormal(frontsector, v1, v2);
 
-		// normal texture
-		gltexture = FMaterial::ValidateTexture(seg->sidedef->GetTexture(side_t::mid), false, true);
-		if (gltexture)
+		if (seg->linedef->skybox == NULL)
 		{
-			DoTexture(RENDERWALL_M1S, seg, (seg->linedef->flags & ML_DONTPEGBOTTOM) > 0,
-				realfront->GetPlaneTexZ(sector_t::ceiling), realfront->GetPlaneTexZ(sector_t::floor),	// must come from the original!
-				fch1, fch2, ffh1, ffh2, 0);
+			// normal texture
+			gltexture = FMaterial::ValidateTexture(seg->sidedef->GetTexture(side_t::mid), false, true);
+			if (gltexture)
+			{
+				DoTexture(RENDERWALL_M1S, seg, (seg->linedef->flags & ML_DONTPEGBOTTOM) > 0,
+					realfront->GetPlaneTexZ(sector_t::ceiling), realfront->GetPlaneTexZ(sector_t::floor),	// must come from the original!
+					fch1, fch2, ffh1, ffh2, 0);
+			}
 		}
 	}
 	else // two sided
diff --git a/src/p_saveg.cpp b/src/p_saveg.cpp
index 7a09b2ee8c..bcbc3deeb7 100644
--- a/src/p_saveg.cpp
+++ b/src/p_saveg.cpp
@@ -478,6 +478,11 @@ void P_SerializeWorld (FArchive &arc)
 		}
 		arc << li->args[1] << li->args[2] << li->args[3] << li->args[4];
 
+		if (SaveVersion >= 4531)
+		{
+			arc << li->skybox;
+		}
+
 		for (j = 0; j < 2; j++)
 		{
 			if (li->sidedef[j] == NULL)
diff --git a/src/p_spec.cpp b/src/p_spec.cpp
index fba5e2a2d9..520586791a 100644
--- a/src/p_spec.cpp
+++ b/src/p_spec.cpp
@@ -1044,6 +1044,23 @@ static void CopyPortal(int sectortag, int plane, ASkyViewpoint *origin, fixed_t
 				}
 			}
 		}
+		if (tolines && lines[j].special == Sector_SetPortal &&
+			lines[j].args[1] == 5 &&
+			lines[j].args[3] == sectortag)
+		{
+			if (lines[j].args[0] == 0)
+			{
+				lines[j].skybox = origin;
+			}
+			else
+			{
+				FLineIdIterator itr(lines[j].args[0]);
+				while ((s = itr.Next()) >= 0)
+				{
+					lines[s].skybox = origin;
+				}
+			}
+		}
 	}
 }
 
diff --git a/src/r_defs.h b/src/r_defs.h
index 0b68bcb8a8..11e4a0e6df 100644
--- a/src/r_defs.h
+++ b/src/r_defs.h
@@ -1057,6 +1057,7 @@ struct line_t
 	sector_t	*frontsector, *backsector;
 	int 		validcount;	// if == validcount, already checked
 	int			locknumber;	// [Dusk] lock number for special
+	TObjPtr<ASkyViewpoint> skybox;
 };
 
 // phares 3/14/98
diff --git a/src/version.h b/src/version.h
index 1ebc9ac5da..875b009df3 100644
--- a/src/version.h
+++ b/src/version.h
@@ -76,7 +76,7 @@ const char *GetVersionString();
 
 // Use 4500 as the base git save version, since it's higher than the
 // SVN revision ever got.
-#define SAVEVER 4530
+#define SAVEVER 4531
 
 #define SAVEVERSTRINGIFY2(x) #x
 #define SAVEVERSTRINGIFY(x) SAVEVERSTRINGIFY2(x)
diff --git a/wadsrc/static/xlat/eternity.txt b/wadsrc/static/xlat/eternity.txt
index ba569393ec..7d52af39c0 100644
--- a/wadsrc/static/xlat/eternity.txt
+++ b/wadsrc/static/xlat/eternity.txt
@@ -39,7 +39,7 @@ define Unsupported (0)
 286 = 0,	Sector_SetPortal(tag,4, 1, 0, 0) // "Portal_HorizonCeiling"
 287 = 0,	Sector_SetPortal(tag,4, 0, 0, 0) // "Portal_HorizonFloor"
 288 = 0,	Sector_SetPortal(tag,4, 2, 0, 0) // "Portal_HorizonFloorCeiling"
-289 = 0,	Unsupported() // "Portal_LineTransfer"
+289 = 0,	Sector_SetPortal(0, 5, 0, tag) // "Portal_LineTransfer"
 
 // Skybox portals
 290 = 0,	Sector_SetPortal(tag, 2, 1, 1, 0) // "Portal_SkyboxCeiling"