Uncle Fester (Addams) is a member of the fictional Addams Family and has been played by numerous actors, beginning with Jackie Coogan in the television series The Addams Family (1964–1966).

Uncle Fester is incorrigible and, except for the good nature of the family and the ignorance of the police, would ordinarily be under lock and key. The complexion, like [that of] Morticia, is dead white, the eyes are pig-like and deeply imbedded, circled unhealthily in black—no teeth and absolutely hairless. He likes to fish but usually employs dynamite or any other unfair means. He keeps falcons on the roof which he uses for hunting. His one costume is a black greatcoat with an enormous collar—summer and winter. He is fat with pudgy little hands and feet.

—  Charles Addams

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 greatcoat. Fester was derived from a character drawn by Charles Addams for a series of cartoons featuring a grotesque family, though Addams never gave the characters names. Nevertheless, the character is recognizable in a number of cartoons, both by his appearance (bald, stooping, sunken eyes) and eerie behavior (turning the shower to a special "scalding" setting, feeding his garden plants blood plasma, and releasing an eagle to attack 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. It is sometimes indicated that he lives in a small shack surrounded by a wrought-iron fence.

Fester has a strange ability to generate electricity. In the film The Addams Family, he loses his memory and regains it after being struck by lightning, leading to him gaining that power, though in other adaptations it is never stated how he got that power. He often demonstrates it by putting a light bulb in his mouth, which illuminates, 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. In the 2022 Netflix series, Wednesday, he shoots lightning directly from his hands and uses it to revive a near-dead Thing. That ability is also believed to give him resistance to lightning, as well as blunt force trauma. When struck directly on the head by a fired cannonball, he appears only mildly dazed, while the cannonball bounces off his head.

Uncle Fester has severe migraines at times but appears to enjoy them. He relieves the 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, he puts his head in the screw press simply for enjoyment.

Despite Uncle Fester's menacing look and bizarre behavior, he is gentle and caring. He shows great respect for Gomez and Morticia, and always exhibits love and concern for his niece and nephews, despite their frequent naughtiness.

In the original 1960s sitcom, Fester is said to be Morticia's maternal uncle (his name rhymes with that of Morticia's mother, Hester). However, from the 1970s onward, he is Gomez's brother. Despite being very close, he admits to sometimes “despising” Gomez and can be jealous of his relationship with Morticia, often trying to find love, with chaotic consequences. In all adaptations, he is either uncle or great-uncle to Wednesday and Pugsley. He is known as "Tío Fétido" in Spain, "Tío Lucas" in other Spanish-speaking countries, and "Fétide" in France. In Brazil, he has two names: "Tio Chico" (in most adaptations) and "Tio Funéreo" (Broadway musical and live-action movies).