The 25-year-old singer went to the Duke of Wellington pub in Chelsea but left behind her usual theatrical outfits and left off the make-up.
She ordered a vodka tonic and chatted with friends and was surprised no one knew who she was.
A source said to UK newspaper Daily Mirror: 'Staff didn't recognise her. She had no make-up on and very normal clothes, just baggy trousers and butch, black, flat shoes. And definitely no wig.
'Gaga was chatting to staff, complimenting one on her tattoo. But within half an hour she was overheard saying: 'This is crazy, I can't believe no one recognises me.'
'Weirdly, she then started making outrageous comments to people passing by and generally drawing attention to herself.'