The Art Museum is, as the name says, a museum with art exhibits in Bikini Bottom. It appears in the episode "Squidward's School For Grown-Ups." Squidward visits it with his new best friend, Patrick. SpongeBob comes along without Squidward and Patrick knowing, because he wants to win Patrick back as his best friend. The only employees known to be employed there are two security guards.
There isn't that much to look at the Art Museum from the outside. It is mainly made of marble and has only one door. Also, it has a short amount of stairs unlike most museums. It's exterior structure looks similar to the Metropolitan Museum of Art in New York City, New York, except it's much smaller.
The inside of the Art Museum is what makes it so popular. It has many paintings and sculptures done by many famous fish. The wall is pretty simple but the floor is pretty shiny as Patrick described that it "compliments" the artwork. The ceiling of the Art Museum also appears to be tiled.
It also makes a brief appearance in "The Masterpiece," where SpongeBob takes a large portion from one of its pillars for Squidward's statue material, causing the museum to collapse. The text looks different, very old-like.
Not counting the security guards, SpongeBob, Patrick, and Squidward were the only ones shown inside the Art Museum.