"Terrible,awful news," tweeted That 70's Show costar Danny Masterson. "Brilliant on 70s.... See u next time LRK,kisses. RT @sdavis53: #RIPLisaRobinKelly aka Laurie Foreman."
Kelly played the older sister of Topher Grace's Eric Forman on the hit Fox series, which ended in 2006.
"I am so sorry to hear about Lisa's passing away," Kurtwood Smith, who played grumpy patriarch Red Forman, said in a statement. "I know the last 10 years have been such a difficult struggle for her but I will always remember the lovely, funny and very talented young lady I worked with."
Kelly's agent, Craig Wyckoff, said in a statement that the actress had been hoping to start a new chapter after years of struggling with substance abuse and related legal issues.
"Lisa had voluntarily checked herself into a treatment facility early this week where she was battling the addiction problems that have plagued her these past few years," he said. "I spoke to her on Monday and she was hopeful and confident, looking forward to putting this part of her life behind her. Last night she lost the battle."
Mackenzie Phillips, who has prominently spoken about her own past struggle with drug abuse, which prematurely ended her run on One Day at a Time back in 1980, tweeted: "Rest in Peace Lisa Robin Kelly. #sososad I didn't know Lisa, but in a way, she is me. And I am her. #ThereIsaSolution."