A close friend who has been helping Lisa and her family tells us, Lisa died Friday at her home in the San Fernando Valley, where she was in hospice care. We're told the cause of death was stomach cancer.

Lisa famously posed for Playboy back in 1980, and the year before she won the first International Federation of Bodybuilders Woman's World Pro Bodybuilding Championship. It was a huge achievement that really put women's bodybuilding on the map.

She became a staple on the TV talk show circuit, also appearing in a slew of health and fitness magazines. She even wrote a book -- "Lisa Lyon's Body Magic" -- which was published in 1981.

She also modeled for iconic photog Robert Mapplethorpe.

Lisa was inducted into the International Fitness and Bodybuilding Federal Hall of Fame back in 2000 ... for helping to elevate bodybuilding into an art form.

And speaking of elevating bodybuilding, Arnold Schwarzenegger told TMZ, "She is the best. I love her."

Lisa was 70.