The couple are big animal rights activists and on Sunday they joined the National Animal Rights Day demonstration in West Hollywood, California.
The vegan actors, who appeared together in Mary Magdalene, wore protest T-shirts while carrying dead birds in their gloved hands on the march organised by Our Planet. Theirs Too.
The charity founded the annual day of observance in 2011 and the couple are supporters.
“They were talking to one another nonstop,” a source told People at the time.
“Small talk between them. Then they went for their car and he’s holding her hand like it was unconscious. Like it fits. They seem a totally natural and easy couple.”
Mara, 34, and Phoenix, 44, have been romantically linked since making the movie together in late 2016, where she played the title character he played Jesus Christ.
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It’s not the first time they have appeared in the same movie though as they both starred in Spike Jonze’s 2013 romantic drama Her.
The couple went public with their romance at the 2017 Cannes Film Festival where his movie You Were Never Really Here was in competition.
Phoenix won the best actor award for his performance in the Lynne Ramsey film and Mara was there to support.
There are rumours that the couple are engaged after she was seen wearing a diamond ring on her engagement finger last month.
Mara was previously in a relationship with Charlie McDowell, the filmmaker son of Malcolm McDowell, while Phoenix has dated Liv Tyler, Topaz Page-Green and DJ Allie Teilz.