Jameis Winston learned the hard way ... star power stops at the shorts -- especially when trying to get into a bar with a strict PANTS ONLY dress code.
The Tampa Bay Buccaneers QB was rejected from the Blue Martini bar in Tampa when he tried to get in Friday night while wearing a pair of shorts ... in front of a sign that said, "Dress code strictly enforced."
We spoke with one of the managers ... who tells us, "It was nothing personal. He was not allowed inside because he did not meet the standards of the dress code."
We're told the bar would ABSOLUTELY welcome Jameis back if he comes around again ... as long as he's got pants.