The SF 49ers rookie sensation is being called out by adult film star Richelle Ryan -- who tells TMZ Sports, "He has a really nice butt."

Of course, porn stars and the 49ers seem to go hand in hand ... just ask QB Jimmy Garoppolo who famously took Kiara Mia on a date in Bev Hills back in 2018. Unfortunately, the relationship didn't last and Jimmy caught hell from prude critics who didn't approve of his lady friend.

As for Bosa, Ryan -- star of classics like "Seduced By a Cougar" -- tells us the 21-year-old ain't too young for her ... "I love 'em young. Young guys are fun. They have energy!"

Oh, and don't worry San Fran ... Ryan says she doesn't want to be a distraction or bring any negative energy toward the team.

"I promise I won't be the curse of the 49ers ... maybe I'll slide in his Instagram DMs."

Good luck, Nick.

Bosa and the 49ers are taking on the Bengals on Sunday in Cincinnati where Nick and his squad are looking to improve their record to 2-0.