Everson -- who founded Planet Muscle magazine -- was a competitive bodybuilder for years who went on to train and marry 6x Ms. Olympia champion Cory Everson, arguably the greatest female bodybuilder of all time. Jeff and Cory divorced back in the '90s.

Sources connected to Jeff told us ... he was found in his bed Monday at his Los Angeles home when his roommate (a former fiancee) returned from a work trip.

We're told it appears Everson may have passed away a few days before his body was discovered. There are no signs of foul play. Officials do not believe Jeff took his own life. We're told investigators believe he died of natural causes.

We spoke with family members who tell us Jeff had recently complained of pain in his legs and feet, but Jeff suspected it was from powerlifting and nothing more serious than that.

Jeff had been open about his steroid use in the past -- but we're told he had not used in a very long time.

Back in his prime, Everson was featured on a bunch of muscle magazines -- both by himself and with Cory.

Jeff met Cory while working at the University of Wisconsin as a strength coach. The two married in 1982 and even launched a bodybuilding-inspired clothing line together, Sampson and Delilah.

Jeff won the Masters Nationals - NPC bodybuilding competition in 1992, the same year he placed 6th in the masters division of the Mr. Universe competition.

R.I.P.