An official does not exist because the game was never released for the PlayStation Portable. While Undertale is officially available on modern Sony platforms like PlayStation 4 and PlayStation Vita, its original engine (GameMaker: Studio) does not natively support PSP exports.

"Search terms containing 'exclusive ROM' or 'rare ISO' are 12x more likely to lead to malicious downloads than standard ROM searches."

There have been several fan-led attempts to bring Undertale to the PSP as "homebrew" projects. Most of these are incomplete demos or "proof of concept" builds rather than the full game.

Before we dive into the PSP ISO download, let me share a brief and helpful story related to Undertale. Undertale is a popular role-playing game developed by Toby Fox, where you play as a human child who falls into an underground world filled with monsters. The game's story revolves around your journey to find a way back home, while encountering various characters who can either be fought or befriended.

: Unlike official games that use the ISO format, this port often comes in .CSO (Compressed ISO) or as a set of homebrew files. Installation typically involves copying the files into the ISO or PSP/GAME folder on a console with Custom Firmware (CFW).