The snake, believed to be a harmless rat snake, eventually slithered away, presumably back into the plumbing, CNN reported.
Bruce Ahlswede told KSAT-TV, a CNN affiliate in San Antonio, he visited Starbucks Tuesday with his wife after a business presentation. He said when he first saw the snake it was on top of the toilet and he thought it was a toy left as a prank until it began moving.
Instead of using the facilities, he went to the manager: "I said 'Hey you've got a snake in your bathroom and she's kind of freaking out.' "
Michele Ahlswede, who joined her husband for a look, got a picture that she posted on Facebook.
The South Texas Herpetology Association sent some expert help to Starbucks at the store's request, but the snake disappeared first. An expert told KSAT the snake was almost certainly a rat snake, a non-venomous constrictor that is generally harmless to humans.