The 2021 Seattle Mariners season was the 45th season in franchise history. The Mariners played their 22nd full season (23rd overall) at T-Mobile Park, their home ballpark in Seattle, Washington. The M's entered this season with the longest active playoff drought in the four major North American professional sports leagues, attempting to make their first postseason in twenty years. Seattle is the only current Major League Baseball (MLB) franchise without a World Series appearance. Despite posting a 90–72 record, their best since the 2003 season, the Mariners were eliminated from postseason contention for the 20th consecutive season after Boston swept the Washington Nationals to clinch the wild card berth and the Yankees clinched the remaining wild card spot on the last day of the season. This season was the second time during their drought that they’d won two more regular season games than the World Series champions.
Regular season
In the Mariners' 1–0 loss to the Los Angeles Dodgers on April 20, the two teams combined for three hits, tying the record for the fewest in an interleague game set by the Boston Red Sox and Montreal Expos in . As that game featured a home run, a double, and a single, and the Mariners/Dodgers games featured just a double and two singles, it broke the record for fewest total bases in a regular-season interleague game.