William Shears Campbell is a fictional Paul McCartney look-alike whose purported existence arose from the fevered efforts of conspiracy theorists to find significance in album photos and hidden musical messages during the Paul is Dead hoax in the late 1960s.

According to this urban legend, a William Campbell won a "Paul look-alike" contest in 1966 and was induced to impersonate Paul after Paul died. In fact such contests were held, but no William Campbell ever won one.

In October 1969, Russ Gibbs , program coordinator for radio station WKNR-FM in Detroit, began a baseless rumor that Paul McCartney had been killed and replaced by a look-alike. Beatles fans scoured the Beatles' albums for hidden "clues" confirming this, and, demonstrating the human capacity to find meaning where no meaning exists, located dozens of "confirming" nuggets of information.

These morsels were concocted together into one more-or-less cohesive tale: that the real Paul, killed by a banana lorry, had been replaced by an actor named either William Campbell or Billy Shears, who had undergone plastic surgery in order to effect a perfect likeness, and who had previously won a Paul McCartney look-alike contest. The letters "OPD," appearing on a costume in a photograph on the Sgt. Pepper's Lonely Hearts Club Band album, were interpreted variously as standing for "Officially Pronounced Dead" and an indication that Billy Campbell had worked in the Ontario Police Department. (The badge on Paul's arm in the Sgt Pepper's album is that of the Ontario Provincial Police. The badge does not contain the lettering "OPD" but rather "OPP". Most Canadians would recognize this at first glance.)

Also on Sgt. Pepper, "Billy Shears" is the name of the lead singer for the fictional Sgt. Pepper's Lonely Hearts Club Band that the Beatles were originally planning to "play" on the album. Though this concept was largely discarded, the end of the title track includes a lyric referring to the "one and only Billy Shears," played by Ringo Starr, who sings the lead vocal on the next song, "With a Little Help from My Friends."