diff --git a/src/playsim/actor.h b/src/playsim/actor.h
index c9465dbbf7..067324988e 100644
--- a/src/playsim/actor.h
+++ b/src/playsim/actor.h
@@ -442,7 +442,9 @@ enum ActorFlag9
 	MF9_DOSHADOWBLOCK			= 0x00000002,	// [inkoalawetrust] Should the monster look for SHADOWBLOCK actors ?
 	MF9_SHADOWBLOCK				= 0x00000004,	// [inkoalawetrust] Actors in the line of fire with this flag trigger the MF_SHADOW aiming penalty.
 	MF9_SHADOWAIMVERT			= 0x00000008,	// [inkoalawetrust] Monster aim is also offset vertically when aiming at shadow actors.
-	MF9_DECOUPLEDANIMATIONS	= 0x00000010,	// [RL0] Decouple model animations from states
+	MF9_DECOUPLEDANIMATIONS		= 0x00000010,	// [RL0] Decouple model animations from states
+	MF9_BLOCKSIGHT				= 0X00000020,	// [inkoalawetrust] This actors' hitbox blocks P_CheckSight().
+	MF9_DOBLOCKSIGHT			= 0x00000040	// [inkoalawetrust] When calling P_CheckSight(), this actor checks for MF9_BLOCKSIGHT actors.
 };
 
 // --- mobj.renderflags ---
diff --git a/src/playsim/p_local.h b/src/playsim/p_local.h
index ea40d386a8..e37b20f525 100644
--- a/src/playsim/p_local.h
+++ b/src/playsim/p_local.h
@@ -249,17 +249,17 @@ extern TArray<spechit_t> spechit;
 extern TArray<spechit_t> portalhit;
 
 
-int	P_TestMobjLocation (AActor *mobj);
-int	P_TestMobjZ (AActor *mobj, bool quick=true, AActor **pOnmobj = NULL);
-bool P_CheckPosition(AActor *thing, const DVector2 &pos, bool actorsonly = false);
-void P_DoMissileDamage(AActor* inflictor, AActor* target);
-bool P_CheckPosition(AActor *thing, const DVector2 &pos, FCheckPosition &tm, bool actorsonly = false);
-AActor	*P_CheckOnmobj (AActor *thing);
+int		P_TestMobjLocation (AActor *mobj);
+int		P_TestMobjZ (AActor *mobj, bool quick=true, AActor **pOnmobj = NULL);
+bool	P_CheckPosition(AActor *thing, const DVector2 &pos, bool actorsonly = false);
+void	P_DoMissileDamage(AActor* inflictor, AActor* target);
+bool	P_CheckPosition(AActor *thing, const DVector2 &pos, FCheckPosition &tm, bool actorsonly = false);
+AActor*	P_CheckOnmobj (AActor *thing);
 void	P_FakeZMovement (AActor *mo);
 bool	P_TryMove(AActor* thing, const DVector2 &pos, int dropoff, const secplane_t * onfloor, FCheckPosition &tm, bool missileCheck = false);
 bool	P_TryMove(AActor* thing, const DVector2 &pos, int dropoff, const secplane_t * onfloor = NULL, bool missilecheck = false);
 
-bool P_CheckMove(AActor *thing, const DVector2 &pos, FCheckPosition& tm, int flags);
+bool	P_CheckMove(AActor *thing, const DVector2 &pos, FCheckPosition& tm, int flags);
 bool	P_CheckMove(AActor *thing, const DVector2 &pos, int flags = 0);
 void	P_ApplyTorque(AActor *mo);
 
@@ -270,7 +270,8 @@ void	P_SlideMove (AActor* mo, const DVector2 &pos, int numsteps);
 bool	P_BounceWall (AActor *mo);
 bool	P_BounceActor (AActor *mo, AActor *BlockingMobj, bool ontop);
 bool    P_ReflectOffActor(AActor* mo, AActor* blocking);
-int	P_CheckSight (AActor *t1, AActor *t2, int flags=0);
+int		P_CheckSight (AActor *t1, AActor *t2, int flags=0);
+AActor* P_CheckForSightBlock(AActor* t1, AActor* t2);
 
 enum ESightFlags
 {
diff --git a/src/playsim/p_sight.cpp b/src/playsim/p_sight.cpp
index 2cf957c8ce..6cf842b9ca 100644
--- a/src/playsim/p_sight.cpp
+++ b/src/playsim/p_sight.cpp
@@ -36,6 +36,7 @@
 
 #include "g_levellocals.h"
 #include "actorinlines.h"
+#include "shadowinlines.h"
 
 static FRandom pr_botchecksight ("BotCheckSight");
 static FRandom pr_checksight ("CheckSight");
@@ -838,6 +839,38 @@ sightcounts[2]++;
 =====================
 */
 
+ETraceStatus CheckForSightBlockers(FTraceResults& res, void* userdata)
+{
+	SightCheckData* output = (SightCheckData*)userdata;
+	if (res.HitType == TRACE_HitActor && res.Actor && (res.Actor->flags9 & MF9_BLOCKSIGHT))
+	{
+		output->HitSightBlocker = res.Actor;
+		return TRACE_Stop;
+	}
+
+	if (res.HitType != TRACE_HitActor)
+	{
+		return TRACE_Stop;
+	}
+
+	return TRACE_Continue;
+}
+
+// [inkoalawetrust] Check if an MF9_BLOCKSIGHT actor is standing between t1 and t2.
+AActor* P_CheckForSightBlock(AActor* t1, AActor* t2)
+{
+	FTraceResults result;
+	SightCheckData SightCheck;
+	SightCheck.HitSightBlocker = nullptr;
+	DVector3 dir = t1->Vec3To(t2);
+	double dist = dir.Length();
+	DVector3 origin = t1->PosPlusZ(t1->Height * 0.75); //Standard sight checks are done at this height too.
+
+	Trace(origin, t1->Sector, dir, dist, ActorFlags::FromInt(0xFFFFFFFF), ML_BLOCKEVERYTHING, t1, result, 0, CheckForSightBlockers, &SightCheck);
+
+	return SightCheck.HitSightBlocker;
+}
+
 int P_CheckSight (AActor *t1, AActor *t2, int flags)
 {
 	SightCycles.Clock();
@@ -904,6 +937,18 @@ sightcounts[0]++;
 		}
 	}
 
+	// [inkoalawetrust] All trivial checks passed, now check for MF9_BLOCKSIGHT
+	if (t1->flags9 & MF9_DOBLOCKSIGHT)
+	{
+		AActor* blocker = P_CheckForSightBlock(t1, t2);
+
+		if (blocker != nullptr)
+		{
+			res = false;
+			goto done;
+		}
+	}
+
 	// An unobstructed LOS is possible.
 	// Now look from eyes of t1 to any part of t2.
 
diff --git a/src/playsim/shadowinlines.h b/src/playsim/shadowinlines.h
index 60caa95069..94777a3f76 100644
--- a/src/playsim/shadowinlines.h
+++ b/src/playsim/shadowinlines.h
@@ -25,9 +25,11 @@ inline FRandom pr_shadowaimz("VerticalShadowAim");
 //
 //==========================================================================
 
+// Generic actor sight check data.
 struct SightCheckData
 {
 	AActor* HitShadow;
+	AActor* HitSightBlocker;
 };
 
 inline ETraceStatus CheckForShadowBlockers(FTraceResults& res, void* userdata)
diff --git a/src/scripting/thingdef_data.cpp b/src/scripting/thingdef_data.cpp
index 997ab78b1a..b1914ea144 100644
--- a/src/scripting/thingdef_data.cpp
+++ b/src/scripting/thingdef_data.cpp
@@ -353,6 +353,8 @@ static FFlagDef ActorFlagDefs[]=
 	DEFINE_FLAG(MF9, SHADOWBLOCK, AActor, flags9),
 	DEFINE_FLAG(MF9, SHADOWAIMVERT, AActor, flags9),
 	DEFINE_FLAG(MF9, DECOUPLEDANIMATIONS, AActor, flags9),
+	DEFINE_FLAG(MF9, BLOCKSIGHT, AActor, flags9),
+	DEFINE_FLAG(MF9, DOBLOCKSIGHT, AActor, flags9),
 
 	// Effect flags
 	DEFINE_FLAG(FX, VISIBILITYPULSE, AActor, effects),
diff --git a/src/scripting/vmthunks_actors.cpp b/src/scripting/vmthunks_actors.cpp
index c102120983..68897b9cd7 100644
--- a/src/scripting/vmthunks_actors.cpp
+++ b/src/scripting/vmthunks_actors.cpp
@@ -1453,6 +1453,13 @@ DEFINE_ACTION_FUNCTION_NATIVE(AActor, CheckSight, P_CheckSight)
 	ACTION_RETURN_BOOL(P_CheckSight(self, target, flags));
 }
 
+DEFINE_ACTION_FUNCTION_NATIVE(AActor, CheckForSightBlocker, P_CheckForSightBlock)
+{
+	PARAM_SELF_PROLOGUE(AActor);
+	PARAM_OBJECT_NOT_NULL(target, AActor);
+	ACTION_RETURN_OBJECT(P_CheckForSightBlock(self, target));
+}
+
 static void GiveSecret(AActor *self, bool printmessage, bool playsound)
 {
 	P_GiveSecret(self->Level, self, printmessage, playsound, -1);
diff --git a/wadsrc/static/zscript/actors/actor.zs b/wadsrc/static/zscript/actors/actor.zs
index 0dc28c396e..1b1b541f4b 100644
--- a/wadsrc/static/zscript/actors/actor.zs
+++ b/wadsrc/static/zscript/actors/actor.zs
@@ -776,6 +776,7 @@ class Actor : Thinker native
 	native Actor, int LineAttack(double angle, double distance, double pitch, int damage, Name damageType, class<Actor> pufftype, int flags = 0, out FTranslatedLineTarget victim = null, double offsetz = 0., double offsetforward = 0., double offsetside = 0.);
 	native bool LineTrace(double angle, double distance, double pitch, int flags = 0, double offsetz = 0., double offsetforward = 0., double offsetside = 0., out FLineTraceData data = null);
 	native bool CheckSight(Actor target, int flags = 0);
+	native Actor CheckForSightBlocker (Actor target);
 	native bool IsVisible(Actor other, bool allaround, LookExParams params = null);
 	native bool, Actor, double PerformShadowChecks (Actor other, Vector3 pos);
 	native bool HitFriend();