Manager Craig Wyckoff says Kelly died on Wednesday at a Los Angeles addiction treatment facility she had entered early this week.
No official cause of death was disclosed.
Kelly portrayed Laurie Forman, sister of Topher Grace's lead character Eric, on the Fox series. It concluded in 2006.
Unlike some of her co-stars - Grace, Ashton Kutcher and Laura Prepon - Kelly fell out of the spotlight after appearing on the sitcom until she started making headlines for personal troubles.
She was arrested in November along with husband Robert Joseph Gilliam in connection with a disturbance at their home in North Carolina.
In June, she was arrested on suspicion of drunken driving after her car stalled in California.
TMZ has further details about her death - watch here:
24.com publishes all comments posted on articles provided that they adhere to our Comments Policy. Should you wish to report a comment for editorial review, please do so by clicking the 'Report Comment' button to the right of each comment.
South Africans can watch funeral TV coverage of the fallen South African sport heroes, soccer star Senzo Meyiwa and Olympic athlete Mbulaeni Mulaudzi this weekend. Read More »
Five of the shows biggest stars will be paid $300 000 an episode. Read More »
New music video!
Did you know?
A scary night!
Guns N' Roses guitarist!
SandtonNew East Consulting Services (Pty) LtdR8 500 - R20 000 Per Month
East RandTower Group (PTY) LtdR12 000 - R14 000 Per Month
TownhousesR 6 900 000
HousesR 18 000 000
HousesR 2 550 000