Manager Scott Servais announced Saturday that Dunn will open the season as Seattle's No. 6 starter, Ryan Divish of The Seattle Times reports.
The 25-year-old was competing with Nick Margevicius for the sixth spot in the rotation and showed enough during spring training to earn the job. Dunn made 10 starts for Seattle last season and had a 4.34 ERA, 1.36 WHIP and 38:31 K:BB over 45.2 innings. Margevicius will begin the season in the bullpen for the Mariners.