William Reinhold "Gatz" Hjortsberg was an American novelist and screenwriter, known for his originality and for writing the screenplay of the film Legend. His novel Falling Angel was the basis for the film Angel Heart (1987). The novel was adapted into an opera in 2015, composed by J. Mark Scearce with a libretto by Lucy Thurber. His 2015 novel Mañana is a thriller set in Mexico.

Hjortsberg was the only child of a Swedish restaurateur father and a Swiss mother. He attended Dartmouth College, the Yale School of Drama (where he met Thomas McGuane), and was a Stegner Fellow at Stanford University. He was married three times, and had a son and a daughter. He died of pancreatic cancer.