Cops in Maryland hit a local bar this weekend after receiving a call from someone claiming 16-year-old Justin Bieber was inside boozing -- but what they found ... was a woman who looked just like him.

TMZ spoke with multiple people inside the "Mug and Mallet" bar in Ocean City who confirm that cops entered on Saturday night looking for an underage drinker with a bowl haircut and a tiny frame.

But the only person who fit that description was a 27-year-old woman named Katie.

Katie tells us she had to whip out ... her ID and prove she wasn't JB -- though she says she gets mistaken for the singer, "all the time!"