John Borland "Jack" Thayer III (December 24, 1894 – September 20, 1945) was an American writer and historian. He was a first-class passenger on the RMS Titanic. He helped many people to solve how the Titanic sank. He later wrote books about the sinking.
Thayer was born on December 24, 1894 in Philadelphia, Pennsylvania. He was married to Lois Cassatt until his death in 1945. Thayer committed suicide on September 20, 1945. He was depressed about his son's death during World War II. He was found in a car in Philadelphia with cuts on his wrist and throat, aged 50.