Martin Smith was 12 months into a 16-year jail sentence after sexually abusing his victim from when she was seven years old.
A Prison Service spokesman said the 46-year-old was found hanging by staff at HMP Manchester on Monday at about 6.30pm.
Attempts to resuscitate him were unsuccessful and he died at 7pm.
Smith’s children Rebecca, five, and Daniel, 11 months, were found dead in a hotel on the Costa Brava in May 2010.
Their mother, Lianne Smith, 43, is awaiting trial in Spain accused of their murder.
Martin was a former Butlins entertainer who appeared as a guest psychic on Living TV’s Most Haunted.
He was found guilty in December 2010 of rape, attempted rape, indecency with a child and indecent assault between May 1995 and May 2004.
At his trial at Liverpool Crown Court, it was revealed Martin would attempt to hypnotise, hit and bully his young female victim, who is now in her 20s, to get her to comply.
Martin and Lianne, who shared a surname but were not married, fled from their home in North Shields, UK in December 2007 after Smith was accused of sex attacks on the girl.
Days after he was extradited back, Lianne was accused of smothering the children with a plastic bag.