The actor's rep tells TMZ ... it all went down in Raleigh, NC Friday -- where the horror icon was expected to attend Carolina Fear Fest ... but shortly before the event kicked off, he was hit from behind during an afternoon walk, leaving him with 4 fractured ribs and a fractured pelvis.

We're told Bill was walking behind his hotel along a dual pedestrian-bike path and that a dude came flying down the lane and ran right into him from behind ... at a speed which we're told was about 30 MPH. Rather than stop to help, Bill's rep says this man bolted ... fleeing the scene.

Bill, as a result, was left writhing in pain ... and we're told help was eventually called for him ... and he was transported to a trauma hospital, where he underwent multiple CT scans.

In the end ... Bill suffered multiple pelvic and rib fractures -- something he noted himself on IG, posting a couple shots of himself in a hospital gown, and eventually being wheeled out in a wheelchair. While what happened to him was serious ... he's keeping his spirits up.

As for the biker who hopped back on his bike and rode off, Moseley -- who's also starred in 2007's "Halloween" threw in a horror character reference, saying ... "And here I thought Jason packed a punch!"

Bill's since been discharged from the hospital, and has a 6-week rehab stay ahead of him, and has been ordered by docs to avoid putting weight on his left leg at all costs.

The folks behind Carolina Fear Fest also confirmed Bill's expected to make a full recovery, and have announced anyone who purchased a photo op with Bill will receive an immediate refund.

Bill's undoubtedly loved and respected in the horror community. In addition to starring in the 1986 sequel to "The Texas Chainsaw Massacre," he also appeared in "Texas Chainsaw 3D" in 2013 as Drayton Sawyer.

He's also a staple in Rob Zombie's "Firefly" trilogy, among countless other horror projects solidifying his status as a genre icon.