The 33-year-old man accused of murdering a beloved Brooklyn philanthropist had a sexual relationship with the man known to locals as “the Santa Claus of the block,” a high-ranking source told The Post.

Dondre Richardson — a registered sex offender who was on parole since being released from prison early 2019 — allegedly stabbed L. Antonio Litman to death and burned down Litman’s Brooklyn brownstone last week, according to cops.

Richardson, who lives in a homeless shelter on Wards Island, has been arrested 17 times on a range of charges, including robbery, assault on a correction officer and possession of a controlled dangerous substance, authorities said.

Richardson sodomized a woman in East Harlem in 2006. He pleaded guilty to criminal sexual act and was sentenced to 14 years in prison, according to authorities. He was released Jan. 15, 2019, records show.

Richardson now faces charges of murder and arson.

Litman was found in the lobby of the Adelphi Street building, which housed his charity Virginia’s House of Hope, when smoke eaters responded to the fire at 3:30 a.m. on Jan. 20, authorities said.

The 55-year-old was stabbed multiple times and died before the fire was started, according to sources.

Cops declared his death a homicide the next day.

Richardson was hospitalized until Wednesday afternoon, according to police, who said he has not yet been arraigned.