William, Willie, Bill, or Billy Thomas may refer to:
Literature
- William Moy Thomas (1828–1910), English journalist, literary editor and novelist
- William Thomas (Islwyn) (1832–1878), Welsh poet
- William Thomas (Gwilym Marles) (1834–1879), Welsh poet
- William Hannibal Thomas (1843–1935), American teacher, journalist, judge, writer and legislator
- William Beach Thomas (1868–1957), British author and journalist
- William F. Thomas (1924–2014), American newspaper editor
- William C. Thomas (1903–1984), American film producer
- Willie B. Thomas (1912–1977), American blues musician
- Bill Thomas (costume designer) (1921–2000), American costume designer
- William Thomas Jr. (child actor) (a.k.a. Billie "Buckwheat" Thomas, 1931–1980), American actor
- Willie Thomas (trumpeter) (1931–2019), American jazz trumpeter and educator
- William Thomas Jr. (1947–2020), American actor
- Bill Thomas (actor) (born 1952), English actor
- Billy Thomas (drummer) (born 1953), American drummer with McBride & the Ride
- William Thomas (actor) (fl. since 1974), Welsh actor
U.K.
- William Thomas (MP for Old Sarum and Downton) (died 1554), Welsh MP for Old Sarum and Downton
- William Thomas (MP for Caernarvonshire) (died 1586), English politician
- William Thomas (MP for Carnarvon) (died 1653), Welsh politician
- William Thomas (MP for Ludgershall) (1630–1686), English lawyer and politician
- Sir William Thomas, 1st Baronet (1641–1706), English politician
- Sir William Thomas, 2nd Baronet (died 1777), a Thomas baronet of Yapton
- Sir William Thomas, 4th Baronet (1777–1850), a Thomas baronet of Yapton
- Sir William Thomas, 5th Baronet (1807–1867), a Thomas baronet of Yapton
- William Stanley Russell Thomas (1896–1957), British politician
U.S.
- William C. E. Thomas (1818–1876), American civic leader
- William H. Thomas (Wisconsin politician) (1821–1898), American politician
- William W. Thomas Jr. (1839–1927), American diplomat and politician from Maine
- William Hannibal Thomas (1843–1935), American teacher, journalist, judge, writer and legislator
- William Reuben Thomas (1866–1943), American civic leader
- W. Aubrey Thomas (1866–1951), US Representative from Ohio
- William M. Thomas (fl. 1868–1876), African-American politician in South Carolina
- William D. Thomas (1880–1936), American politician
- William Kernahan Thomas (1911–2001), American judge
- William Henry Thomas (1920–2009), American politician from Maryland
- Bill Thomas (Montana politician) (born 1935), American politician in Montana House of Representatives
- Bill Thomas (born 1941), US Representative from California
- Bill Thomas (writer) (born 1943), American legal and political author
- Bill Thomas (Alaska politician) (born 1947), American politician in Alaska House of Representatives
- William L. Thomas (born 1967), American jurist