Bautista fouled off a pitch from the Rangers' Tommy Hunter in the seventh inning, sending the ball back behind home plate. It knocked the "B" from the sign honoring Robinson's retired No. 42 off the stadium facing. It fell about 30 feet into the stands.
A man who identified himself as Ian, a Blue Jays season-ticket holder, was hit in the shoulder by the letter. He was sore, but otherwise uninjured.
"I was very surprised, because I didn't expect a 'B' to fall on me," he told The Associated Press.
The letter rolled down the aisle and was picked up by another fan before the stadium staff retrieved it.
The unusual happening in the stands went unnoticed in the dugout.
"I didn't see it at all," said Toronto manager Cito Gaston.