diff --git a/src/p_checkposition.h b/src/p_checkposition.h
index 98891b87c4..c475e282d5 100644
--- a/src/p_checkposition.h
+++ b/src/p_checkposition.h
@@ -36,6 +36,7 @@ struct FCheckPosition
 	TMap<AActor*, bool> LastRipped;
 	bool			DoRipping;
 	bool			portalstep;
+	int				portalgroup;
 
 	int				PushTime;
 
diff --git a/src/p_map.cpp b/src/p_map.cpp
index 0aa3b58ad2..86444a8f88 100644
--- a/src/p_map.cpp
+++ b/src/p_map.cpp
@@ -161,6 +161,7 @@ DEFINE_FIELD_X(FCheckPosition, FCheckPosition, ceilingline);
 DEFINE_FIELD_X(FCheckPosition, FCheckPosition, stepthing);
 DEFINE_FIELD_X(FCheckPosition, FCheckPosition, DoRipping);
 DEFINE_FIELD_X(FCheckPosition, FCheckPosition, portalstep);
+DEFINE_FIELD_X(FCheckPosition, FCheckPosition, portalgroup);
 DEFINE_FIELD_X(FCheckPosition, FCheckPosition, PushTime);
 
 //==========================================================================
@@ -288,6 +289,7 @@ static bool PIT_FindFloorCeiling(FMultiBlockLinesIterator &mit, FMultiBlockLines
 		}
 	}
 
+	// If we are stepping through a portal the line's opening must be checked, regardless of the NOFLOOR flag
 	if (!(flags & FFCF_NOFLOOR))
 	{
 		if (open.bottom > tmf.floorz)
@@ -905,6 +907,7 @@ bool PIT_CheckLine(FMultiBlockLinesIterator &mit, FMultiBlockLinesIterator::Chec
 				tm.floorpic = cres.line->sidedef[0]->GetTexture(side_t::mid);
 				tm.floorterrain = 0;
 				tm.portalstep = true;
+				tm.portalgroup = cres.line->frontsector->GetOppositePortalGroup(sector_t::ceiling);
 				return true;
 			}
 		}
@@ -999,6 +1002,7 @@ bool PIT_CheckLine(FMultiBlockLinesIterator &mit, FMultiBlockLinesIterator::Chec
 			{
 				// Actor is stepping through a portal.
 				tm.portalstep = true;
+				tm.portalgroup = tm.thing->Sector->GetOppositePortalGroup(sector_t::ceiling);
 				return true;
 			}
 		}
@@ -1051,7 +1055,8 @@ bool PIT_CheckLine(FMultiBlockLinesIterator &mit, FMultiBlockLinesIterator::Chec
 		}
 	}
 
-	if (!(cres.portalflags & FFCF_NOFLOOR))
+	// If we are stepping through a portal the line's opening must be checked, regardless of the NOFLOOR flag
+	if (!(cres.portalflags & FFCF_NOFLOOR) || (tm.portalstep && open.bottomsec->PortalGroup == tm.portalgroup))
 	{
 		if (open.bottom > tm.floorz)
 		{
diff --git a/wadsrc/static/zscript/actor.txt b/wadsrc/static/zscript/actor.txt
index 27ce423a81..9dce485db1 100644
--- a/wadsrc/static/zscript/actor.txt
+++ b/wadsrc/static/zscript/actor.txt
@@ -22,6 +22,7 @@ struct FCheckPosition
 	native Actor		stepthing;
 	native bool			DoRipping;
 	native bool			portalstep;
+	native int			portalgroup;
 
 	native int			PushTime;