Thaddeus (Greek Θαδδαῖος, Thaddaios, from Aramaic תדי, Taddai / Aday) is a male given name. It means a heart, or courageous heart.
As of the 1990 Census, 'Thaddeus' was the 611th most popular male name in the United States and 'Thad' was the 846th most popular.
- Thad Luckinbill, American actor on the soap opera The Young and the Restless.
- Taddeo Gaddi (c. 1300–1366), an Italian painter, active during the early Renaissance
- Taddeo di Bartolo, a Sienese painter active up to 1422.
- Taddeo Crivelli (fl. 1451, died by 1479), also known as Taddeo da Ferrara, painter of illuminated manuscripts of the Ferrara school.
- Taddeo Zuccari or Zuccaro (1529–1566), an Italian painter, member of the Roman school of Mannerism
- Tadeusz Kościuszko (1746–1817), a Polish and American Revolutionary General and national hero.
- Tadeusz Romer (1894-1978), a Polish diplomat and politician
- Thaddeus Stevens (1792–1868), leader of the Radical Republicans during the American Civil War and Reconstruction
- Francisco Tadeo Calomarde y Arría (1775–1842), a Spanish statesman
- José Tadeo Monagas (1784–1868), a Venezuelan President and hero of the Venezuelan War of Independence
- Thaddeus S. C. Lowe (1831–1913), American Civil War scientist and inventor
- Thaddeus Coleman Pound (1833–1914), Lieutenant Governor of Wisconsin and grandfather of Ezra Pound
- Thad Jones (1923–1986), an American jazz trumpeter and bandleader
- Thad Cochran (1937–), United States Senator for Mississippi
- Thaddeus McCotter (1965–), former member of the United States House of Representatives from Michigan's 11th District, was chairman of the Republican House Policy Committee and member of the House Financial Services Committee.
- Thaddeus "Toddy" Livingston, Jamaican father of Neville "Bunny Wailer" Livingston. Thaddeus had a relationship with Bob Marley's mother Cedella Booker, which resulted in the birth of a daughter, Pearl. Bob and Bunny lived together during this period and became as close as brothers. They formed Bob Marley And The Wailers together with Peter Tosh later on.
- Tadeo Gomez, an American musician
- Thad Matta, head men's basketball coach at Ohio State University
- Thaddeus Young, American basketball player for the Philadelphia 76ers.
- Thaddeus Cahill, inventor of the telharmonium
- Thad Lewis, an American football quarterback for the Buffalo Bills.
- Tadeusz Soplica, the titular character of Adam Mickiewicz's epic poem Pan Tadeusz, the national epic of Poland.
- Tadzio, the young boy in Thomas Mann's novella Death in Venice.
- Doctor Thaddeus Venture, a lead character in television show The Venture Bros.
- Thaddeus "Curly" Gammelthorpe, a character in television show Hey Arnold!.
- Thaddeus is likely the real first name of The Chief from Get Smart
- General Thaddeus E. "Thunderbolt" Ross is an antagonist of the Hulk. He appears in the 2003 film Hulk and in the 2008 reboot The Incredible Hulk as well as various animated television series.
- Thaddeus Griffin, evil twin brother of Peter Griffin in the television show Family Guy.
- Thaddeus Montgomery, birth name of the Infantata in season one of the FX horror television series American Horror Story
- Thaddeus Beaumont, fictional author in Stephen King's The Dark Half.
- Col. Thaddeus Gearhart, character in the movie The Hallelujah Trail played by Burt Lancaster in 1965
- J. Thaddeus Toad, character in the Disney movie The Wind in the Willows
- Thaddeus Bradley, character in the movie Now You See Me played by Morgan Freeman is a former magician who debunks other magician's tricks
- Clayton Thaddeus "Thad" Greenwood, part time deputy on the TV show Gunsmoke played by Roger Ewing