Dr. Dre Joins Anderson .Paak at L.A.'s FireAid Benefit Concert

L.A. could never forget about Dr. Dre ... the legendary rapper surprised fans at Thursday night's FireAid benefit concert by joining Anderson .Paak on stage!

Check out the video ... he sent the crowd into roaring cheers when he joined the "Leave the Door Open" hitmaker at the Kia Forum to perform his 1999 track "Still D.R.E." -- plus a rendition of his and Tupac’s 1995 hit "California Love."

Dre told fans that performing at the show -- which was put on to support victims of the devastating L.A. wildfires -- was "magical" and thanked them for all their support throughout his 40 years in the business. He's frequently spoken about how growing up in L.A., specifically Compton, has shaped his music career.

The 7-time Grammy winner also shouted out the firefighters and first responders who were on the front lines of the fires, adding ... "It’s all about love for me today."