Uncle Fester, also known as Fester Addams, is a member of the fictional Addams Family. He was played by Jackie Coogan in the original television series.

Uncle Fester is a completely hairless, hunched, and barrel-shaped man with dark, sunken eyes and often a deranged smile. He always wears a heavy, full-length fur coat. Fester was derived from a character drawn by cartoonist , although these were single page cartoons, with no stories or character names. Nevertheless, the character is recognizable in a number of cartoons, both by his appearance (bald, stooping, sunken eyes) and behavior (e.g. turning the shower to a special "scalding" setting, feeding his garden plants blood plasma, and releasing an eagle on the neighbor's homing pigeons). While he is occasionally seen with the rest of the family, particularly on anthology covers, he is also seen on his own more often than the others. He is sometimes indicated to live in a small shack surrounded by wrought-iron fence.

Fester has a strange ability to generate electricity. He would often demonstrate this by putting a light bulb in his mouth, which would illuminate, accompanied by a loud, crackling noise. He claims to possess 110 volts of power in one episode of the sitcom, while in another episode he demonstrated his "magnetic" quality by levitating a metal paperweight up to his hand. When struck directly on the head by a fired cannonball, he appears only mildly dazed, while the cannonball bounces off his head.

Uncle Fester at times has severe migraines, but appears to enjoy them. Fester relieves his migraines by placing his head in a large screw press and tightening it to levels that normal people would not be able to withstand. At times, Fester uses the screw press on his head simply for enjoyment.

Despite Uncle Fester's menacing look and bizarre behavior, he is gentle and caring to everyone. He shows great respect to Gomez and Morticia. He has always exhibited love and great care to his niece and nephews, despite their frequent naughtiness.

In the 1960s television series, Fester is uncle to . In one episode, he became perplexed when asked his last name, suggesting he has somehow forgotten it. In several episodes, Fester refers to the Addams lineage as if it were his own, but the flashbacks in the episodes "Morticia's Romance, Parts 1 and 2," clearly establish him as Morticia's uncle, brother of her mother, Hester Frump, a.k.a. Granny Frump (Margaret Hamilton), whose maiden name was similarly unrevealed. Perhaps, lacking a last name of his own, Fester was all the more dedicated to upholding the honor of another.

Fester's known ancestry dates back to his Great-Grandfather Blob (not to be confused with Gomez's Cousin Blob, a two- headed ghost), who received the gift of a sacred ruby after he had pried it from the head of a Hindu (to whom it was giving a terrible headache). The ruby remained in the family until Fester unthinkingly used his slingshot to hurl it at a yowling stray cat. "Well, it was the only rock in the house!" he said in his defense.

Per the 1960s sitcom, little is known of Fester's childhood, save that his father (Morticia's maternal grandfather) was an excruciatingly strict man who severely disciplined him, paddling him even when he was good and paying him to stay out of public (Fester considered this to be experience in "public relations"). He refused to allow Fester to even touch a battleaxe (a treasured toy among people with the Addams's macabre tastes) until he was eight. As an adult, Fester defended his father's strictness, pointing to his own character as proof of its effectiveness: "I didn't become what I am by accident! I had upbringing like no other!" Some time prior to Morticia's marriage to Gomez, Fester worked as a newspaper columnist, writing advice for the lovelorn, but left that job because people kept suing him.

Fester has also offered contradictory information about his educational background. In the sitcom's first episode, he notes that he never went to school ("And look how [well] I turned out!"), but he later claims to have failed recess three times. As an adult, Fester took correspondence courses in various subjects, and his educational endeavors occasionally formed an episode's main plot. It may be from one such course that he obtained his fraternity paddle, which he once threatened to use on Wednesday in imitation of his father's punishment style; however, like many an uncle, Fester proved to have more bark than bite in dealing with the children. When he suspects someone of maligning, cheating, or otherwise mistreating anyone in the family, Fester is ever ready with his blunderbuss "Genevieve", eager to "shoot 'em in the back!" However, he is far less eager in meeting a malefactor in a face-to-face duel; he was briefly enthusiastic about a pistol duel with an enemy until he asked, somewhat timidly, "Does he get one too?...Loaded?"

Another prized possession was his cannon, "Old Reliable", which he normally kept in his bedroom. Fester maintained a treehouse in the Addams yard and frequently retreated to his closet to think. After receiving the gift of a motorcycle in the episode " Meets a Beatnik", Fester often drove it through the living room, inevitably crashing in the conservatory (Several episodes used identical footage of this scenario.). Fester also collects three-dollar bills.