Foster, 50, made the revelation while accepting a lifetime achievement award, which her longtime friend, actor Robert Downey Jr., presented to her on behalf of the Hollywood Foreign Press Association.
She has long declined to speak about her private life to the media, but chose the big night to address speculation about her sexuality.
After thanking her many collaborators and friends from over the years, Foster said with a smile: "While I'm feeling all confessional, I just have a sudden urge to say something I haven't ever really been able to air in public that I'm a little nervous about, but maybe not quite as nervous as my publicist ... .
"But I'm just going to put it out there. Loud and proud, right? And I'm going to need your support. I am single," she said to laughter. "Yes, I am! I am single. I'm kidding, but not really. Thank you for the enthusiasm. Can I get a wolf whistle or something?"
Her two young sons beamed with pride and her close pal, actor-director Mel Gibson, gave her a wolf whistle from their table in front of the stage, as the crowd applauded her.
She went on to say she didn't plan to make a "big coming out speech tonight."
"Because I already did my coming out about 1,000 years ago, back in the Stone Age," Foster said. "In those very quaint days when a fragile young girl would open up to trusted friends and family and co-workers and then gradually, proudly to everyone who knew her, everyone she actually met. But now, apparently, I'm told, every celebrity is expected to honor the details of their private life with a press conference, a fragrance and a prime-time reality show. You guys might be surprised, but I am not Honey Boo Boo Child. I'm sorry; that's not me and it never was and it never will be. ... But, seriously, if you had been a public figure from the time you were a toddler, if you had to fight for a life that felt real and honest and normal against all odds, then maybe you, too, would value privacy against all else."
Foster also recognized her ex-girlfriend, Cydney Bernard, as "one of the deepest loves of my life."
"My heroic co-parent; my ex-partner in love, but my righteous soul sister in life," she said. "I am so proud of our modern family, our amazing sons -- Charlie and Kit -- who are my reason to breathe and evolve. My blood and soul. And boys, in case you didn't know it, this song, all of this, is for you."
Foster's films include "Taxi Driver," "Silence of the Lambs" and "The Accused."