Mayo, short for Mayonnaise, is a sauce made of olive oil and eggs. It's sometimes used on a Krabby Patty. It first appears in the episode "Nature Pants."
- In "Nature Pants," SpongeBob gives Patrick a Mayo pot.
- In "Hooky," according to Mr. Krabs, the only perspective of being in a Tuna can is the smell of mayo.
- In "Mermaid Man and Barnacle Boy II," SpongeBob has trouble opening a jar of mayo. He uses the Conch Signal to call Mermaid Man and Barnacle Boy to help him.
- In "Band Geeks," Squidward reveals that mayonnaise is not an instrument, after Patrick asks if it is. This has become a popular meme on the Internet.
- In "Just One Bite," Harold has a lot of mayo on his patty and Squidward thinks it is disgusting.
- In "Hocus Pocus," SpongeBob turned Patrick into mayo, after Patrick asked him to.
- In "Planet of the Jellyfish," the Jellions' weakness is mayo. As a result, the replicas are not fond of it. SpongeBob and Sandy made use of mayo to defeat the Jellions and the Evil Alien Jellyfish Overlord.
- In The SpongeBob Movie: Sponge Out of Water, mayo is used as a weapon against Plankton's Tank Robot and for a strange reason all the customers want to hold the Mayo from the Krabby Patty.