William "Billy" Lyall (26 March 1953 – 1 December 1989) was a Scottish musician.
Born in Edinburgh, Scotland, Lyall was a keyboard player, vocalist and occasional flautist with Pilot, and an early member of the Bay City Rollers.
He was keyboard player for Dollar between 1978 & 1982, and also contributed to The Alan Parsons Project with fellow Pilot members.
He left Pilot in early 1976 and released a solo album, Solo Casting later that year.
He also contributed string arrangements and synthesizer parts to an album by the band Runner (Island records) in 1979.Lyall died of AIDS-related causes in 1989, aged 36.
Bay City Rollers' manager Tam Paton later said that Lyall was gay.