Games like Upstairs
Games like Upstairs include Garry's Mod, PEAK, Gang Beasts, R.E.P.O., and Grand Theft Auto V Legacy. We matched them by the tags players give Upstairs on Steam (Difficult, Walking Simulator, Multiplayer, Singleplayer, and Physics): closest matches first, and among equally close ones the better-known games.

Garry's Mod
Shared: Multiplayer · Physics · Singleplayer

PEAK
Shared: Multiplayer · Physics · 3D

Gang Beasts
Shared: Multiplayer · Physics · Funny

R.E.P.O.
Shared: Multiplayer · Physics · Comedy

Grand Theft Auto V Legacy
Shared: Multiplayer · Third Person · Singleplayer

RV There Yet?
Shared: Multiplayer · Physics · Funny

ELDEN RING
Shared: Difficult · Multiplayer · Singleplayer

Bloons TD 6
Shared: Multiplayer · Singleplayer · 3D

Human Fall Flat
Shared: Multiplayer · Physics · Funny

Monster Hunter: World
Shared: Multiplayer · Difficult · Third Person

Baby Steps
Shared: Walking Simulator · Comedy · Third Person

Getting Over It with Bennett Foddy
Shared: Difficult · Singleplayer · Physics

I Am Bread
Shared: Difficult · Physics · Singleplayer

Super Battle Golf
Shared: Multiplayer · Physics · Comedy

Just Cause™ 3
Shared: Multiplayer · Singleplayer · Action

Chained Together
Shared: Multiplayer · 3D Platformer · Casual

Crash Bandicoot™ 4: It’s About Time
Shared: Difficult · 3D Platformer · Singleplayer

Octodad: Dadliest Catch
Shared: Physics · Walking Simulator · Comedy

Get To Work
Shared: Difficult · Singleplayer · 3D Platformer

SpongeBob SquarePants: Battle for Bikini Bottom - Rehydrated
Shared: Multiplayer · 3D Platformer · Singleplayer

Warhammer 40,000: Space Marine 2
Shared: Multiplayer · Singleplayer · Third Person

Grand Theft Auto IV: The Complete Edition
Shared: Multiplayer · Singleplayer · Third Person

Who's Your Daddy?!
Shared: Multiplayer · Funny · Comedy

Fireworks Mania - An Explosive Simulator
Shared: Multiplayer · Singleplayer · Physics