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 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.
- 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.