Few details were immediately available about Simmons' death, which was confirmed to ABC News by his representative on Saturday.
The Los Angeles Police Department responded to his home after a 911 call from his housekeeper and found Simmons dead, police sources told ABC News. He appears to have died of natural causes, and no foul play is suspected, the sources said.
Simmons had celebrated his 76th birthday on Friday.
Simmons had spooked fans just three months ago when he posted that he was dying on Facebook, writing in March, "I have some news to tell you. Please don't be sad. I am .... dying. Oh I can see your faces now. The truth is we all are dying. Every day we live we are getting closer to our death."
He later clarified that he was not actually dying, but that the message was intended to encourage everyone to "embrace every day that we have."
Simmons has been out of the public eye lately, though there were reports of a biopic being made about the fitness enthusiast earlier this year. Simmons criticized the reports, writing, "Don't believe everything you read. I no longer have a manager, and I no longer have a publicist. I just try to live a quiet life and be peaceful."
ABC News' Bonnie McLean contributed to this report.