Miley Cyrus had a butt-naked birthday celebration -- latching on to a stripper's bare ass, and TMZ has the first pic of the awesome encounter ... inspired by none other than 2 Chainz!
Miley rang in her 20th last month with a house party in Hollywood ... and sources inside the bash tell us Miley's friends ordered a few strippers -- one of whom dropped a booty-shaking lap dance on the birthday girl.
We're told Miley couldn't resist (we understand) ... jumped up, hugged the dancer and grabbed her ample butt, which was busy swallowing a pink g-string.
Miley issued a warning tweet before her birthday ... "if I don't get atleast one big booty hoe my friends are officially not my friends anymore" -- a clear nod to "Birthday Song" by 2 Chainz.
Even with Miley's hands-on approach ... her stripper-palooza was totally G-rated compared to her bf Liam Hemsworth's last birthday cake.