EXCLUSIVE

Derek Fisher told police someone broke into his L.A. home and stole more than $300,000 worth of valuable jewelry ... including his 5 NBA Championship rings ... TMZ Sports has learned.

Law enforcement sources tell us the NBA legend reported a burglary at his home Monday morning -- telling police he left the home at 7:30 AM and when he returned 3 hours later, it was clear someone had been inside.

We're told officials believe the perp got into the house through a side door -- and went for his jewelry. Among the stolen goods was Fisher's 5 NBA rings he won during his run with the Lakers. They're each worth a ton of cash.

Our sources say other jewelry was taken as well.

The LAPD is currently investigating. So far, no arrests have been made.