The Joker is a fictional DC Comics villain, widely considered to be Batman's arch-enemy and the most well-known villain in comic book history. The idea was conceived by Batman inker\letterer Jerry Robinson (while on staff under Batman creator\artist Bob Kane), the Joker first appeared in Batman #1 (1940). Originally conceived as an evil "court-jester" type, the character was rejected by studio writer Bill Finger as being "too clownish" however, he later relayed the idea to Bob Kane. Kane, who started out as a "gag" artist, loved the concept and encouraged its production. Mr. Finger found a photograph of actor Conrad Veidt wearing make-up for a silent film entitled The Man Who Laughs (which was based on a novel by Victor Hugo) and it was from this photograph that the Joker was modelled.

The Joker is a humor-obsessed criminal with a clown-like appearance, who likes to kill people with fatal hilarity. Writers have alternatively portrayed him as a goofy trickster-thief or as a serial killer with a warped sense of humor. Recent writers of the Batman comic book series have preferred the latter, and the Joker has been responsible for numerous tragedies in Batman's life, such as the murder of Jason Todd, the successor to the mantle of Robin after Dick Grayson, the injury/paralysis of Barbara Gordon, a.k.a. Batgirl/Oracle, and the death of Sarah Essen Gordon, Jim Gordon's wife.

The Joker also killed Alexander Luthor, the hero of the original Crisis on Infinite Earths and villain of its sequal Infinite Crisis.

The Joker is regarded by fans and comic-book aficionados as one of the greatest villains in comic-book history and by many the one with the most psychological depth.

The Joker has been featured throughout the Batman saga, and has been an enemy of the Caped Crusader in most adaptations in other media. Interpretations of the Joker that have made him well-known to the general public include Cesar Romero's in the 1960s Batman television series, Mark Hamill in the DC Animated Universe and Jack Nicholson's in the 1989 feature film.

The Joker (as played by Nicholson) ranks number 45 in the American Film Institute's list of the top 50 film villains of all time.