Joseph "Joe" Fitzpatrick Fitzgerald Fitzsimmons Fitzhenry Quimby, Jr., a.k.a. "Diamond Joe" Quimby, or simply Mayor Quimby (voiced by Dan Castellaneta), is a fictional character featured in The Simpsons animated series. He has long served as the Democratic mayor of the fictional city of Springfield. He appears to be a slick, sleazy politician whose only interest lies in staying in power, the fringe benefits power brings and womanising. Those benefits apparently include squandering tax dollars for personal use, receiving bribes from mobster Fat Tony, and gaining access to vast hordes of nubile young women.

Joe Quimby's name partly comes from NW Quimby Street in Portland, Oregon, the hometown of show creator Matt Groening. The rest of the character's name, as well as his character, is from Portland mobster and chief of police, "Diamond" Jim Purcell. It is also interesting to note that the producer of the Tom and Jerry cartoons (the model for The Itchy & Scratchy Show on the Simpsons), is Fred Quimby. Mayor Quimby's nephew is named Freddy.

Quimby is a caricature of one of the members of the Kennedy family, who have long been active in American politics. He speaks with an eastern Massachusetts accent, in a style resembling that of the Kennedy family. The mayor is apparently a Democrat, like the Kennedys, comes from sizable wealth, and lives on a coastal estate (the "Quimby compound") with his family—not unlike the Kennedy compound in Hyannisport, Massachusetts. Quimby is married to a woman who wears a pink suit and pillbox hat similar to one Jacqueline Kennedy made famous. She may be also related to actor Rainier Wolfcastle, who is modeled after Arnold Schwarzenegger (husband of JFK's niece, Maria Shriver); Wolfcastle has been spotted at Quimby family functions. The mayor's nephew, Freddy Quimby, was once tried for assault (a reference to the William Kennedy Smith rape trial) during which Mayor Quimby tried to bribe witnesses. Mayor Quimby does not get along with local Police Chief Clancy Wiggum. Few other members of the Quimby clan have been specified, although Lisa refers to Joe's brother Clovis as the "beer-swilling" driver who ran over the Simpson family cat Snowball (I).

He is known to be a womanizer, and to occasionally amuse himself with pornographic playing cards during town meetings. He frequently takes overseas vacations, which take him out of town for extended periods of time, leading to the headline "Mayor Visits City" in the Springfield Shopper. He once referred to Springfield as Springfeld at a public meeting.

Quimby's corrupt personality and love of overseas junkets is perhaps most amusingly highlighted during one episode where he announces that after a three month "fact finding mission", he has concluded that it is NOT feasible to build a high speed rail link between Springfield and the tropical paradise resort of Aruba.

It has been implied on occasion that he is connected to Fat Tony, the local Mafia Don. It is certain that Quimby will bend the law if it suits his purposes: he once arranged Marge Simpson's release after she was arrested for blocking traffic on a bridge after suffering a nervous breakdown, claiming that if she went to jail, Quimby could kiss the "chick vote" goodbye. He also resorted to bribing witnesses when assault charges were brought against his nephew, leading Bart Simpson to observe that "the system works: just ask Claus Von Bulow."