Yoob's Belly (Japanese: ゲッシーの中 Gesshi's Insides) is a location in the game Mario & Luigi: Partners in Time. During the events of the game, Mario, Luigi, Baby Mario, and Baby Luigi found Kamek and Baby Bowser on Yoshi's Island's mountaintop, possessing the Cobalt Star Shards they needed. After beating Kamek, they pursued Baby Bowser, who had ran off. But while trying to get the Cobalt Star Shards (which Baby Bowser had eaten), a few Shroob UFOs had passed by and noticed them. They headed off and made Yoob (who was climbing the mountain) grow humongous (even more so). He was now big enough to see the group, and he proceeded to eat Baby Bowser and the four Mario Bros. Now inside the Yoob, the brothers had to find a way out, and at the same time find Baby Bowser and rescue the Yoshis. After rescuing a few Yoshis, they managed to push a Chomp Rock down a small passage in Yoob, widening it and allowing them to proceed.
Eventually, they found out that Yoob also has a bio-engineered factory inside of itself, which traps Yoshis in Yoob eggs. This will soon transform them into Yoobs if they aren't freed in time. The four soon find a Toad named Toadbert. He was on the Time Machine alongside Princess Peach and Toadiko, but can't remember a single thing about what happened to her, except for one thing, which he had previously depicted through a picture of Peach battling Princess Shroob. The Brothers proceed to the end, where they find Sunnycide, who is blocking their exit and looking after the Yoob eggs. After beating the monstrosity, the passage opens, and the Yoshis, Toadbert, Baby Bowser and the four Mario Brothers exit. Yoob, having its source of energy (the Yoob eggs) gone, "deactivates" and just lies on the mountainside. The group can go back into Yoob's Belly anytime in the game in case they missed something or simply wish to explore the stomach yet again.