Coach Mike Zimmer made the announcement Monday.
Cassel has been sharp while starting all three preseason games. He has completed more than 66 percent of his passes for 367 yards with two touchdowns and one interception. He opened training camp in competition with Bridgewater, one of the team's two first-round draft picks, for the starting job.
Bridgewater has completed 65 percent of his passes for 266 yards and four touchdowns in the preseason. But most of his game time has come with the second team.
The Associated Press contributed to this report.