Drew Ditlefsen, 27,told the Minneapolis Star Tribune he heard the glass shatter and then smelled the fumes. Flames spread across the pavement behind him.
"It was just like, Whoosh," he said.
Ditlefsen said he stopped about 30 yards away and saw two people standing on the bridge. He said he heard one of them say "Let's get out of here!" and they ran away.
He didn't get a good look at them and police were investigating the 1 p.m. attack, the Tribune said.
Minneapolis police sent out arson and explosives experts to investigate the 1 p.m. attack.
Ditlefsen, who often uses his bike to pull a trailer loaded with coffee beans for his job with Peace Coffee, said he's had sticks and bottles thrown at him from overpasses before, and knows someone whose had a grocery cart lobbed down on them, but the gas bomb was a first.
He suggests fences be installed along the overpasses to prevent more attacks, the Tribune said.