no official version PlayStation Portable (PSP) . Because the game was developed in GameMaker: Studio, it was never natively released for that hardware.

What About Homebrew?

The PSP homebrew community once attempted to port Undertale via GameMaker Player for PSP, but performance was abysmal (single-digit FPS). No stable, playable ISO exists. Any site claiming a “hot, working Undertale PSP ISO” is misleading you.

: A project created for a PSP Game Jam that provides a playable demo of the game's mechanics on the handheld. This can often be found on community platforms like Chapter 1 & 2 "Files"

If you're interested in playing Undertale on a portable console, consider the following alternatives:

Strictly speaking, there is no official Undertale ISO for the PSP. Most "hot downloads" you find online claiming to be a full ISO are often misleading or potentially harmful.