The "Lethal Weapon" and "Braveheart" star, who has admitted he is an alcoholic, has been criticized in recent years for making racist and anti-Semitic remarks. He also is under investigation for domestic violence after his ex-girlfriend claimed he threatened and assaulted her.
However, Foster said she remains loyal to the actor, who she says is going through a "dark moment," TVGuide.com said.
"When you love a friend, you don't abandon them when they are struggling," Foster said in an interview for the October issue of More magazine. "Of course, Mel is an undeniably gifted actor and director ... but more importantly, he is and has been a true and loyal friend. I hope I can help him get through this dark moment."
Foster and Gibson collaborated on 1994's "Maverick" and the soon-to-be-released "The Beaver."
Describing Gibson as the "easiest, nicest person I've ever worked with," the actress added: "The second I met him, I said, 'I will love this man for the rest of my life.'"