Upon use, creates a Red Room next to a wall where there would be no conditions that could prevent a room from spawning there (i.e., the same conditions for a Secret Room to spawn). This is indicated by red door outlines on walls.
All rooms made by Red Key will be 1x1 rooms that can have doors available on all sides.
All doors within a Red Room will be unlocked or opened, regardless of the room type they lead to. This can open Secret Rooms, and normally inaccessible Challenge Rooms if they are adjacent to a Red Room. Red Room doors will also stay permanently open in the case of Challenge Rooms.
In Home, Isaac can create a "Red Room" on the left side of the map right under Mom's room. This room will actually be a closet containing a non-tainted character's Tainted form.
If the respective Tainted character has already been found or a Tainted character is played, the room will contain Inner Child instead. If Inner Child has not been unlocked, the room will contain a Shopkeeper.
This can also be done by using a Cracked Key, which can be obtained through room drops or by leaving a trinket in a boss or item room and coming back to that room during the Ascent, and using it in the same spot.
If a door created by Red Key takes Isaac outside of the 13x13 area of the floor, he will be taken to the I AM ERROR room.
The starting room of a floor is always in the center of the 13x13 area.
Additionally, this doesn't work during Red Redemption (challenge #44).
The I AM ERROR room door can only be created while already inside a Red Room. Normal rooms will not show the red outline if they are at the map's boundary.
Special Red Rooms can be versions that aren't normally encountered without a specific item or character.
Red Key can't overwrite Secret Rooms or Super Secret Rooms. This can be used to discern their locations by finding walls that don't have red door outlines that would otherwise be valid Red Room spots.
Red Key can't create a room adjacent to the Boss Room on most floors, with some exceptions:
Red Key can create a room adjacent to Hush's room in the top right where the room that leads to The Void usually generates. The door to this looks like a crack. This Red Room can only be created when Isaac is in another Red Room adjacent to it, i.e. from branching off one of the Treasure Room.
Isaac can enter Hush's room from this crack, but going the other direction will lead him to the Entrance Portal room.
Isaac also can't create Red Rooms adjacent to the Shop on this floor either.
Red Key can create rooms adjacent to most Boss Rooms in The Void, including Delirium's room. This can be used to find a Boss Room door leading to what would be an empty space (though this may find Mr. Fred instead).
When Isaac enters any Red Room adjacent to the Ultra Secret Room, it immediately appears on the map along with the Secret Room jingle, and its door will automatically open.
If Isaac encounters a Red special room, all of its doors will be its special theme doors.
All doors in a Red Curse Room still feature spikes, which will damage Isaac if he decides to enter/exit.
A Red Vault's doors or a Red Dice Room's doors each require two keys to open. The door where Isaac uses Red Key will be automatically unlocked, but the other doors remain locked until Isaac goes through them from inside or unlocks them from outside.
Multiple Red special rooms of the same type can spawn on one floor.
All Red Sacrifice Rooms will have their own independent use counters and rewards.
If Isaac has a teleport to a specific room (XVII - The Stars, IX - The Hermit, Teleport 2.0, etc.) and has created a special room such that there are multiple of a certain type on one floor, the teleport will choose randomly between each room.
However, if Isaac has found a Red Planetarium, XVII - The Stars will always teleport Isaac to it over any Treasure Rooms.
Red Rooms in Downpour/Dross can contain White Fire Places as well.
Isaac is able to create Red Rooms in the Mirrored World of Downpour / Dross II. They will be reflections of the original floor's Red Rooms, and special rooms created in this way will still be empty.
When in Dark Room or Chest, their spawn room containing their respective chests can be created by Red Key. These will function the same way regular chests of their respective type would on Dark Room/Chest outside the starting room.
New Mega Satan doors can't be found in these rooms.
If a red Sacrifice Room appears in the Dark Room and Isaac teleports via the 12th or higher sacrifice, Isaac will be sent back to the starting room without restarting the floor.
If a red Sacrifice Room appears in The Void, then it is possible to defeat The Lamb after taking the path through Chest. This makes it possible to get up to 8 Completion Marks in the same run if playing quickly.
Found Soul: If Found Soul is positioned against a different wall, activating the Red Key will open two red rooms at once: One near Isaac, and one near Found Soul.
Interactions
4.5 Volt: Usually generates fewer than 1 charge per room, as Red Rooms are always 1x1.
Red Room next to a wall where there would be no conditions that could prevent a room from spawning there (i.e., the same conditions for a 
Secret Room to spawn). This is indicated by red door outlines on walls.
Devil Rooms or 
Angel Rooms.
Challenge Rooms if they are adjacent to a Red Room. Red Room doors will also stay permanently open in the case of Challenge Rooms.
Home, Isaac can create a "Red Room" on the left side of the map right under Mom's room. This room will actually be a closet containing a non-tainted character's Tainted form.
Inner Child instead. If Inner Child has not been unlocked, the room will contain a 
Shopkeeper.
Cracked Key, which can be obtained through room drops or by leaving a trinket in a boss or item room and coming back to that room during the Ascent, and using it in the same spot.
I AM ERROR room.
Corpse II, 
Dark Room, 
Chest, 
The Void, or the special version of 
Mausoleum / 
Gehenna II containing 
Dad's Note. No door outlines will appear at any boundary walls.
Tainted Keeper's 
Shops, which can additionally contain items from the Boss and Treasure Room pools, and offer trinkets for sale.
Voodoo Head's 
Curse Rooms.
Cain's 
Birthright 
Arcades.
More Options' double item 
Treasure Rooms.
Pay to Win's 
Shop Restock Machine 
Super Secret Rooms. This can be used to discern their locations by finding walls that don't have red door outlines that would otherwise be valid Red Room spots.
Boss Room on most floors, with some exceptions:
Hush's room in the top right where the room that leads to 
Delirium's room. This can be used to find a Boss Room door leading to what would be an empty space (though this may find 
Mr. Fred instead).
Downpour / 
Dross II.
Curse of the Labyrinth takes effect on the floor.
Ultra Secret Room, it immediately appears on the map along with the Secret Room jingle, and its door will automatically open.
Vault's doors or a Red 
Dice Room's doors each require two keys to open. The door where Isaac uses Red Key will be automatically unlocked, but the other doors remain locked until Isaac goes through them from inside or unlocks them from outside.
Sacrifice Rooms will have their own independent use counters and rewards.
XVII - The Stars, 
IX - The Hermit, 
Teleport 2.0, etc.) and has created a special room such that there are multiple of a certain type on one floor, the teleport will choose randomly between each room.
Planetarium, XVII - The Stars will always teleport Isaac to it over any 
Knife Piece 1 outside of the intended method by spawning the 
Mine Cart Room of 
Mines / 
Ashpit II.
Mega Satan doors can't be found in these rooms.
The Lamb after taking the path through 
Pandora's Box at 
Greed and 
Super Greed can spawn in red 
Mini-Boss Rooms.
Bag of Crafting with eight 
Book of Virtues: Creates a Red Key wisp in the middle ring. The wisps have a chance to create Red Rooms when Isaac enters an unexplored room.
Broken Glasses / 
Devil's Crown / 
Judas' Tongue / 
Keeper's Bargain / 
Number Magnet / 
A Pound of Flesh / 
Sanguine Bond: Affects Red 
Luna: Affects Red 
Mama Mega!: Will trigger its effect in new Red Rooms, provided it has already been used on the floor.
Spelunker Hat: Instantly reveals the 
Found Soul: If Found Soul is positioned against a different wall, activating the Red Key will open two red rooms at once: One near Isaac, and one near Found Soul.
4.5 Volt: Usually generates fewer than 1 charge per room, as Red Rooms are always 1x1.
Car Battery: No effect.
Member Card: The trap door will always lead to the same special room as the regular 
Mother) as 
Keys 
The Binding of Isaac: Rebirth
