Yoob is a Shroob monster that debuted in Mario & Luigi: Partners in Time. He first enters Yoshi's Island as a massive egg the Yoshis used as a tourist attraction, charging five coins to view it. Eventually, Yoob's egg hatches. Yoob proceeds to eat what Yoshis he can, leaving the others to take shelter in a nearby hut.

Shortly thereafter, Mario, Luigi, Baby Mario and Baby Luigi arrive on Yoshi's Island in the past. They find it deserted. Yoob finds the hidden Yoshis and proceeds to eat the last few survivors. He then walks away, into the mountains.

Later, after the Mario Bros. defeat Kamek and find Baby Bowser, Yoob is struck with an energy beam from a Shroob UFO. The energy causes Yoob to mutate into a much larger dinosaur. Yoob climbs the large cliff and eats the Mario Bros. and Baby Bowser, who had stolen the Mario Bros. Cobalt Star pieces.

Inside Yoob's Belly, the Mario Bros. find that it's not organic; rather, it's a biomechanical factory. Inside, Shroobs torture the Yoshis that are imprisoned inside, and then imprison them in Yoob eggs, which resemble purple Yoshi Eggs with green spots. From these eggs Yoobs would hatch if left alone.

After rescuing the Yoshis, they push a Chomp Rock into Yoob's intestines. Once in the center of Yoob's internal factory, the Mario Bros. battled the guardian of the Yoob Factory, an egg monster called Sunnycide. After defeating it and rescuing the Yoshis, Yoob powers down into a state of deactivation.

If the Mario Bros. return to the cliff later, they can find Yoob's body still clinging to the cliff. They can reenter it and look for items they may have missed the first time they went through the level.

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