William or Billy Pratt may refer to:

• William Abbott Pratt (1818–1879), American photographer
• William B. Pratt (1935–2019), American physician and politician
• William Pratt (MP for Southwark), English politician
• William Henry Pratt, real name of actor Boris Karloff
• William Pratt (born 1609), ancestor of the Pratt-Romney family
• William Pratt (rugby league) (1932–2009), English rugby league footballer of the 1950s for Leeds, and Halifax
• William K. Pratt, American educator, businessman, and author
• William V. Pratt (1869–1957), admiral in the United States Navy
• William Pratt (cricketer) (1895–1974), English cricketer
• William Dymock Pratt (1854–1916), English architect
• William Pratt (businessman) (1928–1999), Canadian businessman
• Billy Pratt (footballer, born 1872) (1872–?), English football winger for Small Heath from 1889 to 1892
• Billy Pratt (footballer, born 1874) (1874–?), English football full-back for Small Heath from 1894 to 1902
• Spike (Buffy the Vampire Slayer), a character from television series Buffy the Vampire Slayer