Hallett starred in more than 70 episodes of the show as demon Lorne
Andy Hallett, a singer who gained fame portraying a green-skinned demon on the cult US TV series Angel, has died of congestive heart disease aged 33.
Hallett was taken to Cedars-Sinai Medical Center in Los Angeles after having problems breathing and died there on Sunday, his agent said.
It follows a five-year battle with the heart condition.
The star appeared on more than 70 episodes of the Buffy the Vampire Slayer spinoff between 2000 and 2004.
Born and raised in Osterville, Massachusetts, Hallett moved to LA when he was 23 and worked as a messenger at a talent agency.
He went on to become an assistant to the wife of Buffy the Vampire Slayer creator Joss Whedon - who had Hallett audition for the part of Lorne on Angel after seeing him perform karaoke.
Hallett proved himself a fan favourite on the show as the show tune loving, red-horned demon who could read a person's aura when they sang, revealing their problems and futures.
The series, which starred Bones actor David Boreanaz as the brooding vampire Angel, ran for five seasons before it was cancelled in 2004.
Hallett was diagnosed with his heart condition at the end of the show and had been in and out of hospitals for the past five years.
He decided to concentrate on his own music, stepping away from acting, agent Pat Brady said.
Hallett, an only child, is survived by his father Dave Hallett and mother Lori Hallett.
A private funeral service will be held for family and close friends in Cape Cod, Massachusetts, this weekend.