Mariners claim relief pitcher Isaac Mattson from Pittsburgh Pirates

The Seattle Mariners picked up a bullpen option Monday, acquiring right-handed pitcher Isaac Mattson on a waiver claim from the Pittsburgh Pirates.

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Mattson, 31, had a 5.35 ERA and 1.60 WHIP in 38 appearances for Pittsburgh in 2026, striking out 36 batters while walking 18 over 38 2/3 innings.

In 2025, Mattson was a standout arm for the Pirates, finishing the season with a 2.45 ERA, 1.13 WHIP and 45 strikeouts to 19 walks over 44 appearances (47 2/3 innings).

Per Baseball Savant, Mattson averaged 94.2 mph on his four-seam fastball in 2026, and also features a sweeper, changeup and slider. He heavily relies on his fastball, throwing it at least 70% of the time in each of the past three seasons.

The 6-foot-2 Mattson’s extension on his throwing motion has ranked in the top 70th percentile in each of his four MLB seasons, per Baseball Savant.

A Pennsylvania native and University of Pittsburgh product, Mattson was a 19th round pick in the 2017 MLB Draft by the Los Angeles Angels. He debuted in 2021 with four appearances for the Baltimore Orioles, then didn’t break back into the big leagues until 2024 with the Pirates.

Monday will be a busy day for transactions in baseball, as teams have to return players on their 60-day injured list to the 40-man roster five days after the conclusion of the World Series, which occurred last Wednesday, Oct. 28. Monday is also the deadline for teams to extend qualifying offers to pending free agents, as free agency officially begins at 2 p.m.

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