Ratatouille is a 3D platform game based on the Pixar animated film of the same name. Unlike versions for other platforms, the PSP game does not follow the plot of the film but instead presents its own story set after the film's climax, with the voice actors from the film reprising their roles for the game.

Shortly before Linguini and Collete can open their new bistro, their recipes are stolen by their disgruntled ex-employer Skinner. It falls to gourmet rat Remy to bring back the recipes so the new restaurant can open in time.

Remy must move through five worlds of six to seven levels each, accessible from the rat colony hub world. The worlds include Linguini's apartment, the sewers, the marketplace, Paris rooftops and the restaurant. In most levels, Remy's goal is to pick up the recipe that lies somewhere in a hard-to-reach place, but other levels consist only of races or boss fights and the recipe is awarded upon winning. To get around, Remy can jump and double-jump, pull himself up ledges, crouch and run on all fours to get into tight spaces, climb up vertical and swing along horizontal poles and also swim and dive. Some enemy critters like spiders, caterpillars and bats want to hinder Remy on his quest, but he can punch them or hit them with his tail to defeat them. Some small puzzles must be solved to overcome some obstacles, usually involving finding a certain object to be used at the correct place. The ghost of chef Gusteau usually offers hints on what to do next, and Remy's fellow rats also sometimes offer helpful advice.

Lying about the levels are various collectibles. Apples and cupcakes restore lost health. Pieces of cheese function as currency and can be traded in with the merchant rat in the rat colony for upgrades: increasing Remy's health bar, adding more combat moves, increasing his swimming speed and more. Hidden in most levels are star keys. These can be used to unlock chests in the rat colony that contain additional recipes not found in the regular levels. Found recipes can be looked at in the game menu and contain actual cooking instructions for various dishes.

The game also includes two multiplayer modes: a race and a cheese collecting game. The race optionally allows placing chili peppers on the track, the eating of which increases speed. The cheese collecting game has several variants. In the regular variant, the player who collects the most cheese within the time limit wins. The 'weighted' variant makes a player slow down upon collecting cheese. Putting the collected cheese into a designated drop area restores regular speed. In the 'chef hat' variant, hats materialize when five pieces of cheese have been collected, with the winner being the player who collects the required number of hats or the most hats after the time limit ends.