diff --git a/src/actionspecials.h b/src/actionspecials.h
index 7e783051e..bf6cc38b5 100644
--- a/src/actionspecials.h
+++ b/src/actionspecials.h
@@ -105,6 +105,7 @@ DEFINE_SPECIAL(Scroll_Texture_Down, 103, -1, -1, 2)
 DEFINE_SPECIAL(Ceiling_CrushAndRaiseSilentDist, 104, 3, 5, 5)
 DEFINE_SPECIAL(Door_WaitRaise, 105, 4, 5, 5)
 DEFINE_SPECIAL(Door_WaitClose, 106, 4, 5, 5)
+DEFINE_SPECIAL(Line_SetPortalTarget, 107, 2, 2, 2)
 
 DEFINE_SPECIAL(Light_ForceLightning, 109, 1, 1, 1)
 DEFINE_SPECIAL(Light_RaiseByValue, 110, 2, 2, 2)
diff --git a/src/p_lnspec.cpp b/src/p_lnspec.cpp
index c74378f61..0061fd5dc 100644
--- a/src/p_lnspec.cpp
+++ b/src/p_lnspec.cpp
@@ -57,6 +57,7 @@
 #include "d_net.h"
 #include "d_event.h"
 #include "gstrings.h"
+#include "portal.h"
 #include "r_data/colormaps.h"
 #include "fragglescript/t_fs.h"
 
@@ -3383,6 +3384,11 @@ FUNC(LS_Thing_SetConversation)
 	return true;
 }
 
+FUNC(LS_Line_SetPortalTarget)
+// Line_SetPortalTarget(thisid, destid)
+{
+	return P_ChangePortal(ln, arg0, arg1);
+}
 
 static lnSpecFunc LineSpecials[] =
 {
@@ -3493,7 +3499,7 @@ static lnSpecFunc LineSpecials[] =
 	/* 104 */ LS_Ceiling_CrushAndRaiseSilentDist,
 	/* 105 */ LS_Door_WaitRaise,
 	/* 106 */ LS_Door_WaitClose,
-	/* 107 */ LS_NOP,
+	/* 107 */ LS_Line_SetPortalTarget,
 	/* 108 */ LS_NOP,
 	/* 109 */ LS_Light_ForceLightning,
 	/* 110 */ LS_Light_RaiseByValue,
diff --git a/src/portal.cpp b/src/portal.cpp
index bfb90cd14..fbbe5c95c 100644
--- a/src/portal.cpp
+++ b/src/portal.cpp
@@ -29,6 +29,24 @@ FArchive &operator<< (FArchive &arc, FLinePortal &port)
 }
 
 
+static line_t *FindDestination(line_t *src, int tag)
+{
+	if (tag)
+	{
+		int lineno = -1;
+		FLineIdIterator it(tag);
+
+		while ((lineno = it.Next()) >= 0)
+		{
+			if (&lines[lineno] != src)
+			{
+				return &lines[lineno];
+			}
+		}
+	}
+	return NULL;
+}
+
 void P_SpawnLinePortal(line_t* line)
 {
 	// portal destination is special argument #0
@@ -36,21 +54,7 @@ void P_SpawnLinePortal(line_t* line)
 
 	if (line->args[2] >= PORTT_VISUAL && line->args[2] <= PORTT_LINKED)
 	{
-		if (line->args[0] > 0)
-		{
-			int linenum = -1;
-
-			for (int i = 0; i < numlines; i++)
-			{
-				if (&lines[i] == line)
-					continue;
-				if (tagManager.LineHasID(&lines[i], line->args[0]))
-				{
-					dst = &lines[i];
-					break;
-				}
-			}
-		}
+		dst = FindDestination(line, line->args[0]);
 
 		line->portalindex = linePortals.Reserve(1);
 		FLinePortal *port = &linePortals.Last();
@@ -156,6 +160,45 @@ void P_FinalizePortals()
 	}
 }
 
+static bool ChangePortalLine(line_t *line, int destid)
+{
+	if (line->portalindex >= linePortals.Size()) return false;
+	FLinePortal *port = &linePortals[line->portalindex];
+	if (port->mType == PORTT_LINKED) return false;	// linked portals cannot be changed.
+	port->mDestination = FindDestination(line, destid);
+	if (port->mType == PORTT_INTERACTIVE)
+	{
+		FLinePortal *portd = &linePortals[port->mDestination->portalindex];
+		if (portd != NULL && portd->mType == PORTT_INTERACTIVE && portd->mDestination == line)
+		{
+			// this is a 2-way interactive portal
+			port->mFlags = port->mDefFlags | PORTF_INTERACTIVE;
+			portd->mFlags = portd->mDefFlags | PORTF_INTERACTIVE;
+		}
+		else
+		{
+			port->mFlags = port->mDefFlags;
+			portd->mFlags = portd->mDefFlags;
+		}
+	}
+	return true;
+}
+
+
+bool P_ChangePortal(line_t *ln, int thisid, int destid)
+{
+	int lineno;
+
+	if (thisid == 0) return ChangePortalLine(ln, destid);
+	FLineIdIterator it(thisid);
+	bool res = false;
+	while ((lineno = it.Next()) >= 0)
+	{
+		res |= ChangePortalLine(&lines[lineno], destid);
+	}
+	return res;
+}
+
 // [ZZ] lots of floats here to avoid overflowing a lot
 bool P_IntersectLines(fixed_t o1x, fixed_t o1y, fixed_t p1x, fixed_t p1y,
 				      fixed_t o2x, fixed_t o2y, fixed_t p2x, fixed_t p2y,
diff --git a/src/portal.h b/src/portal.h
index 6a947fd0e..53fb1c16e 100644
--- a/src/portal.h
+++ b/src/portal.h
@@ -52,6 +52,7 @@ extern TArray<FLinePortal> linePortals;
 
 void P_SpawnLinePortal(line_t* line);
 void P_FinalizePortals();
+bool P_ChangePortal(line_t *ln, int thisid, int destid);
 
 
 /* code ported from prototype */