The 2025 Seattle Mariners season was the 49th season in franchise history and their 26th full season (27th overall) at T-Mobile Park, their home ballpark in Seattle, Washington. The Mariners won their first American League (AL) West divisional title and Division Series since 2001.

Catcher Cal Raleigh had an unprecedented home run hitting season, finishing the regular season with 60 home runs. He broke the franchise record for home runs (set by Ken Griffey Jr.) and major league records for home runs by a catcher and switch hitter in a season. He also won the Home Run Derby, which Griffey won three times.

Raleigh, Randy Arozarena, Julio Rodríguez, Bryan Woo, and Andrés Muñoz were selected to the All-Star Game. Raleigh was named the AL Player of the Month for June, and Muñoz was named the AL Reliever of the Month for March/April. Raleigh won the Silver Slugger Award, and he and Rodríguez were named to the All-MLB Team. Woo and Muñoz were named to the All-MLB second team.

On August 9, the Mariners retired the number 51 worn by Ichiro Suzuki, following his induction into the National Baseball Hall of Fame in July. This was the fourth number retired by the team. The Mariners played in the MLB Little League Classic on August 17, losing to the New York Mets.

The Mariners clinched a postseason berth for the first time since 2022 on September 23, with a victory over the Colorado Rockies. The next night, the team won the AL West division for the fourth time in franchise history.

The Mariners defeated the sixth seed Detroit Tigers in the AL Division Series (ALDS). With their win in Game 2, the Mariners won their first home postseason game since Game 5 in the 2001 ALDS. The Mariners won the series in 5 games, reaching the AL Championship Series for the first time since 2001, where they faced the Toronto Blue Jays. Despite winning three of the first five games, the Mariners squandered 2–0 and 3–2 series leads, losing in Game 7 after allowing a go-ahead home run to George Springer. The Mariners extended their drought of never winning the AL pennant.

The Mariners started poorly, going 4–8 to begin the season, including being swept by the San Francisco Giants. Starting with an 8th inning grand slam by Randy Arozarena on April 9, the team's performance improved enough to lead the AL West at the end of the month. The team won their last seven series of the month. Rowdy Tellez hit home runs in three consecutive games at Rogers Centre, his former home ballpark, to help win a series against the Toronto Blue Jays. The Mariners then defeated Boston Red Sox ace Garrett Crochet in another series win. Several players had career-best performances, with Dylan Moore and Jorge Polanco winning back-to-back AL Player of the Week awards. Their offense, along with Cal Raleigh's 10 home runs, carried the Mariners. Closer Andrés Muñoz tied for the MLB lead with 11 saves and did not allow a run, winning the AL Reliever of the Month Award.

The team was beset by injuries: starting pitcher George Kirby and relievers Matt Brash and Troy Taylor started the season on the injured list, and regulars Víctor Robles, Logan Gilbert, Ryan Bliss, Moore, Luke Raley, and Gregory Santos ended April injured. Polanco also dealt with a knee injury that prevented him from switch hitting or playing defense. As part of the team's reinforcements, Ben Williamson, Logan Evans, and Sauryn Lao all made their major league debuts.