About
Undertale Yellow is a free, fan-made prequel to Toby Fox's Undertale, following the journey of Clover, the first human to fall into the Underground before the events of the original game. Developed over approximately seven years by a community team led by Master Sword, the full game released on December 9, 2023 to widespread praise from the Undertale fanbase. Built in GameMaker: Studio, it features original music from noted composers including NoteBlock, and expands meaningfully on the lore and world of Undertale. It is widely considered one of the highest-quality Undertale fan projects ever released.
Identity
Launch
- Binary
- Undertale Yellow.exe
- Needs files
- none beyond the binary
Proton
- Version
- proton_experimental
- Winetricks
- —
⚙ Setup notes
Build: Windows GameMaker build; runs under Proton on Steam Deck.
Install: Download the ZIP (the page ships a .zip, e.g. Undertale Yellow v1_1PatchFr213.zip — there is no installer) from the itch.io page (taediumbreak.itch.io/undertale-yellow), extract it, and add the extracted Undertale Yellow.exe (note the space in the filename) as a non-Steam game.
Proton: Set Proton Experimental in Properties > Compatibility. Community reports confirm it runs without problems under Proton Experimental.
Controller: The GameMaker build has only partial built-in gamepad support (analog-stick mapping is awkward). For a clean Deck experience, configure a controller layout in Steam Input — e.g. map the left stick / D-pad to the arrow keys and face buttons to Z/X/C — rather than relying on the game's native pad handling.
Note: If the .exe name in the extracted archive differs from a future build, use the main .exe at the root of the extracted folder.
The one thing to know
Install: Download the ZIP release from itch.io (the only Windows download — there is no installer). Add the extracted Undertale Yellow.exe (filename contains a space) as a non-Steam game and force Proton Experimental.
No owned data: No owned data files required; fully self-contained free game.
Unofficial: Unofficial Undertale fan prequel — not affiliated with Toby Fox or Fangamer.
Community guides
Write-ups and threads from people who got this (or a similar) game running. deckport links to them — it doesn't reproduce them.
Get the artwork
deckport never hosts game images. Open this game on SteamGridDB, pick
the cover / hero / logo / icon you like, and drop them into the
game folder under .deckport-art/ before you push it to the
Deck. The importer files them under the right names automatically.
Run it on your Deck
Two files: the one-time importer (deckport.py) in your
Deck's home folder, and this game's install helper. Copy the game
into ~/Games and run the helper with Steam closed — it
writes the recipe (binary, launch options, Proton version)
and registers the shortcut with artwork.