Joaquin Phoenix and Rooney Mara name first child after actor's late brother, River

The co-stars recently welcomed a 'beautiful son', according to director Viktor Kossakovsky

Rooney Mara and Joaquin Phoenix have welcomed their first child, a baby boy reportedly named after the 'Joker' actor's late brother, River Phoenix. EPA

Joaquin Phoenix has paid a touching tribute to his late brother by naming his firstborn baby River.

The Joker actor, 45, recently welcomed his first child with long-term partner Rooney Mara, 35, according to director Viktor Kossakovsky.

The Russian filmmaker made the revelation at the Zurich Film Festival on the weekend, after a screening of his documentary Gunda, on which Phoenix acted as executive producer.

Explaining why the Oscar-winning actor wasn't at the festival for the occasion, Kossakovsky said Phoenix had just welcomed a "beautiful son called River".

The name is believed to be in honour of the star's brother, River, who died in 1993. The Stand by Me actor passed away at the age of 23 due to a drug overdose.

Phoenix and Mara, who met while filming Her in 2013, are notoriously private, and have not confirmed their child's arrival, nor even the pregnancy.

The Walk The Line actor, who was 19 at the time of River's death, also paid tribute to his brother last year when accepting his Best Actor Oscar.

"When he was 17, my brother wrote this lyric. He said: 'Run to the rescue with love and peace will follow'," said Phoenix.

In an interview earlier this year, the actor also told Anderson Cooper his performances were influenced by his brother.

"I feel like in virtually every movie that I made, there was a connection to River in some way," he said. "And I think that we've all felt his presence and guidance in our lives in numerous ways."