Theodore "Beaver" Cleaver is the fictional title character of the American television series Leave It to Beaver. Originally played by Jerry Mathers, Beaver is the son of June and Ward Cleaver (Barbara Billingsley and Hugh Beaumont) and the brother of Wally Cleaver (Tony Dow).

The Beaver prefers "messin' around" with his pals and reading comic books to attending church or taking dance lessons. Most episodes in the series feature the Beaver getting into trouble at home, in school, or around the neighborhood and then receiving timely and appropriate moral instruction from his father regarding his misbehavior.

Leave It to Beaver was created by the writers Joe Connelly and Bob Mosher, who found inspiration for dialogue and plot lines among their own children. The Beaver was based on Connelly's son Ricky.

Theodore "Beaver" Cleaver was portrayed by Jerry Mathers in the pilot, "It's a Small World"; the original series; the spinoff telemovie, Still the Beaver; and the sequel series, The New Leave It to Beaver. In the reunion telemovie and the sequel series, the Beaver is a divorced father of two children living at home with his widowed mother, June. In the 1997 feature film adaptation of the original series Leave It to Beaver, the Beaver was played by Cameron Finley.

The Cleavers live in the fictional town of Mayfield. Ward is a white-collar, briefcase-toting businessman, whose actual occupation is never mentioned; he states he was an engineer while serving as a Seabee. June is a full-time homemaker. In the second season, Wally is a high-school student. Being four years older than the Beaver, he often acts as a bridge between his parents and his brother. He puts communications from the parents into kidspeak for the Beaver, and keeps his parents apprised of the Beaver's plans, feelings, and activities. The Cleavers dwell in a spacious house on Mapleton Drive during the first two seasons. They moved to a similar house on nearby Pine Street for the remaining four seasons. The family drives a 1957 Ford Custom 300 in the first two seasons. In the remaining seasons, when the Chrysler Corporation was a sponsor of the program, the family car is a Plymouth Belvedere or a Plymouth Fury.

The Beaver has a paternal great uncle, Billy (Edgar Buchanan), who makes a few appearances in the series. Billy, a world traveller, is not entirely trusted by June because he fills her sons' heads with fancies of irresponsible living. The Beaver also has a maternal great aunt, Martha Bronson (Madge Kennedy), who buys the Beaver a short pants suit and wants him to attend a hoity toity prep school on the east coast. The Beaver was named for Martha's brother, Theodore, and in one episode, Martha gives the Beaver Theodore's heirloom ring. The Beaver also has a maternal aunt named Peggy, and an infant cousin (neither of whom appear on the show). No grandparents or other relatives appear on the show, though Ward and June occasionally mention their parents while recalling incidents from their childhoods.

The Beaver's best friends are Larry Mondello, Gilbert Bates, Whitey Whitney, Richard Rickover, and the elderly Gus, a fireman. While the Beaver's peers cannot always be trusted or relied on in times of trouble, Gus can always be counted on to be sympathetic to the Beaver's woes and to steer the boy along the straight and narrow.

The Beaver is sometimes led astray by his school friends. In one episode, for example, Larry Mondello tells the Beaver the principal has a spanking machine in her office and the Beaver steals in to take a peek. He is accidentally locked in the office and calls the fire department to be freed. His parents are furious.