The closest you can get to playing Undertale on a PSP is a fan-made project called . This isn't a direct port of the original game's code, but rather a passionate attempt to recreate the Undertale experience using the Lua programming language and the PSP's homebrew capabilities. It's a work in progress, not a finished game.
Undertale PSP ISO: The Ultimate Guide to Playing on PlayStation Portable undertale psp iso hot download
While enthusiasts continue to work on technical "proofs of concept," these are far from a complete, playable ISO: The closest you can get to playing Undertale
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Because an official ISO doesn't exist, files labeled as such are often one of the following: