Joaquin Phoenix has had a baby son with actress Rooney Mara — naming him in tribute to his late movie star brother River, according to the director of his latest film.

“He just got a baby by the way,” director Victor Kossakovsky told the 2020 Zurich film festival Sunday when discussing the “Joker” star’s involvement as executive producer on his new documentary, “Gunda.”

“A beautiful son called River,” the director said.

Page Six was first to reveal that “The Social Network” star Mara, 35, was expecting a child with the notoriously reclusive 45-year-old actor.

The couple originally met on the set of the Spike Jonze film “Her,” but did not begin dating until they were reunited on the set of the film “Mary Magdalene.” They confirmed their engagement in May last year.

Phoenix has also been open about the devastation caused by losing his 23-year-old brother to a drug overdose at Johnny Depp’s LA nightclub in 1993 — during which he made a harrowing 911 call as he watched on.

“River was a really substantial actor and movie star, and we didn’t really know it,” he recently told Anderson Cooper on “60 Minutes” of his brother famed for movies like “Stand By Me” and “My Own Private Idaho.”

“So during that time in which you’re most vulnerable, there are helicopters flying over. There are people that are trying to sneak on to your land. Certainly, for me, it felt like it impeded on the mourning process,” he said.