The 40-year-old star was asked to sing live on the show after she revealed that she will exercise her vocal cords in a forthcoming film.
Ellen, who is Paula Abdul's replacement on American Idol, managed to persuade Jennifer to perform - after she initially protested.
A new career path?: Jennifer Aniston sings 'I've Got A Crush On You' in front of a live audience on Ellen DeGeneres' chat show
After dimming the lights and handing Jennifer a microphone, Ellen asked the actress what she would sing.
To which Aniston replied: 'I've Got A Crush On You.'
She then launched into the chorus of the song, which became more of a serenade than a solo performance.
When Jennifer finished her short performance Ellen, 51, told her: 'I would put you through to the next round.
See Jennifer's effort and judge for yourself by scrolling down to the bottom of the page.
Jennifer's Got Talent: Actress sings live on Ellen DeGeneres' chat show
I've Got A Crush On You has been performed by Frank Sinatra and 1930s jazz singer Lee Wiley, and although Jennifer's version was not quite up there with such legends, it was passable considering she was taken by surprise.
Aniston faced further embarrassment when she took part in a quiz in which she had to guess whose lips she had kissed.
She was shown a series of lips up close, and had to guess which of her co-stars they belonged to.
After correctly guessing Owen Wilson and Gerard Butler, she floundered with long-term Friends co-star David Schwimmer and Aaron Eckhart, who she stars with in new romantic drama Love Happens.Aniston faced further embarrassment when she took part in a quiz in which she had to guess whose lips she had kissed.
She was shown a series of lips up close, and had to guess which of her co-stars they belonged to.
Guess who: Jennifer Aniston is shocked to find the lips she was shown belong to former Friends co-star David Schwimmer
Meanwhile the 41-year-old actor denied the couple had an on-set romance while shooting the film, but admitted she was a good kisser.
Eckhart appeared on the The Late Show With Conan O'Brien, and revealed: 'I thought we had a nice chemistry, she was wonderful to work with, beautiful, a wonderful actress as well and a good kisser.'
He added: 'I'm getting pressure from the press. When you're promoting a new movie they want to know if you've slept with your co-star, which I have not.'
