William, Willie, Bill, or Billy Thomas may refer to:
Academics
W. I. Thomas (William Isaac Thomas, 1863–1947), American sociologist
William Jenkyn Thomas (1870–1959), Welsh headmaster and author
William Davies Thomas (1889–1954), Welsh literary scholar
William G. Thomas III (born 1964), American historian
William Thomas (historian) (1936–2025), British historian
C. William Thomas, American accounting academic
Arts and entertainment
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
Performing arts
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
Billie "Buckwheat" Thomas (William Thomas Jr., 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
Billie Geter Thomas (William Bryan Geter Thomas, 1902–1962), American educator and actress
Visual arts
William Thomas (architect) (1799–1860), Anglo-Canadian architect
William Tutin Thomas (1829–1892), Anglo-Canadian architect
Bill Thomas (American writer, born 1934) (1934–2009), American author and photojournalist
Business and industry
William Bevil Thomas (1757–1825), Newfoundland merchant and land developer
William Kyffin Thomas (1821–1878), Australian newspaper proprietor
William Luson Thomas (1830–1900), British wood-engraver and newspaper proprietor
William Thomas (miller) (1838–1891), Australian flour mill industrialist
William Winstead Thomas (1848–1904), American insurance company president and architect
Sir William James Thomas, 1st Baronet (1867–1945), Welsh industrialist and philanthropist
Bill Thomas (businessman) (born 1959), British business executive
Law and politics
Australia
William Thomas (Australian settler) (1793–1867), Australian colonist
William Thomas (Victorian politician) (1869–1924), Victorian politician
William Lemen Thomas (1872–1921), Western Australian politician
Bill Thomas (Australian politician) (born 1950), Western Australian politician
UK
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, MP for Caernarvonshire, 1574–86
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
Russell Thomas (politician) (William Stanley Russell Thomas, 1896–1957), British politician
US
W. C. E. Thomas (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 H. Thomas (Alabama judge) (1867–1945), associate justice of the Alabama Supreme Court