The Mariners claimed Mattson off waivers from the Pirates on Monday, Colin Beazley of the Pittsburgh Post-Gazette reports.
Mattson posted a 2.45 ERA and 45:19 K:BB over 47.2 innings in 2025, but he stumbled in 2026 with a 5.35 ERA and 36:18 K:BB over 38.2 frames. The 31-year-old will look to fill a middle relief role in Seattle.