Boston Red Sox vs Seattle Mariners Prediction 6/21/2026 Today’s MLB Picks

In this article, we will formulate a Red Sox vs Mariners Prediction for this MLB matchup on Sunday, June 21st in the third game of this series. Let’s take a look at the prediction for this MLB matchup.

Seattle Mariners Preview

The Seattle Mariners have a 39-39 record this season and are sitting in the 1st place of the AL West. They have a 21-19 home record and are coming off a 1-5 home defeat against the Red Sox. They will face the Pirates and the Guardians next.

The Mariners have a .232 batting average this season, .313 OBP and .388 Slugging percentage. Seattle’s pitching staff has a 3.66 ERA and 1.18 WHIP. Randy Arozarena is the team’s best hitter with a .291 batting average. Julio Rodriguez leads the team in RBI with 37, while Luke Raley adds a team-high 14 home runs.

Logan Gilbert (R) is the projected starting pitcher for the Mariners, and he has a 5-4 record, 3.43 ERA and 1.03 WHIP.

Boston Red Sox Preview

The Boston Red Sox have a 31-43 record this season and are sitting in the 5th place of the AL East. They have a 19-18 road record and are coming off a 5-1 road win against the Mariners. They will face the Rockies and the Yankees next.

The Red Sox have a .244 batting average this season, .312 OBP and .382 Slugging percentage. Boston’s pitching staff has a 3.86 ERA and 1.26 WHIP. Willson Contreras is the team’s best hitter with a .286 batting average, also leading the team in RBI with 44 and adding a team-high 16 home runs.

Payton Tolle (L) is the projected starting pitcher for the Red Sox, and he has a 3-4 record, 2.93 ERA and 1.06 WHIP.

Boston Red Sox @ Seattle Mariners Betting Trends: June 21, 2026

• Probable Starting Pitcher: Logan Gilbert      Record this season: 5-4      ERA: 3.43
• Home Record: 4-3
• Last 5 against Red Sox: 0-2

Why the Seattle Mariners will win

• The Mariners have won 12 of their last 14 games against the Red Sox following a home loss.
• The Red Sox have lost five of their last six day games against AL West opponents.
• The Mariners have covered the run line in each of their last four day games against American League opponents.
• The Red Sox have failed to cover the run line in five of their last six day games against AL West opponents.
• The Mariners have led after 3 innings in 11 of their last 12 day games against American League opponents that held a losing record.
• The Red Sox have lost the first inning in five of their last six road games against AL West opponents.
• The Red Sox have trailed after 5 innings in five of their last six day games.

• Probable Starting Pitcher: Payton Tolle      Record this season: 3-4      ERA: 2.93
• Road Record: 2-2
• Last 5 against Mariners: –

Why the Boston Red Sox will win

• The underdogs have won each of the Red Sox’s last four games.
• The Mariners have lost four of their last five games as favorites after playing the previous day.
• The road team has covered the run line in each of the Red Sox’s last six games.
• The Mariners have failed to cover the run line in each of their last six Sunday games as favorites.
• The Red Sox have led after 5 innings in five of their last six games at T-Mobile Park.

Total Runs Facts

• Four of the Red Sox’s last five games have gone UNDER the total runs line.
• Six of the last eight games between the Red Sox and Mariners at T-Mobile Park have gone UNDER the total runs line.
• There has been at least one run scored in the first inning in eight of the Red Sox’s last nine road games against AL West opponents.
• There has been no run scored during the first inning in each of the last four day games between the Red Sox and Mariners at T-Mobile Park.

Seattle Mariners Player Prop Facts

• Logan Gilbert has recorded a win in four of his last five home appearances against opponents that held a losing record.
• Victor Robles has recorded at least one hit in each of his last 11 appearances in day games against teams that held a losing record.
• Logan Gilbert has recorded seven or more strikeouts in 12 of his last 13 home appearances against AL opponents that held a losing record.
• Patrick Wisdom has hit a home run in two of his last three home appearances against the Red Sox.
• Logan Gilbert is one of only six pitchers to record 90+ Strikeouts and 20 or less Walks this season.

Boston Red Sox Player Prop Facts

• Ceddanne Rafaela has recorded at least one hit in each of his last 10 appearances in day games.
• Triston Casas has hit a home run in two of his last three appearances against the Mariners.

Matchup/League Facts

• The Mariners rank 1st in the league for walks allowed this season (200).
• The Mariners rank 29th in the league for doubles this season (98).
• The Red Sox rank 30th in the league for RBIs this season (280).
• The Red Sox rank T29th in the league for runs scored this season (293).

Red Sox vs Mariners Prediction

The Red Sox have won their last three meetings against the Mariners, all of them on the road.

In this Red Sox vs Mariners, the Mariners are coming as -150 home favorites. I like the pitching in this matchup, as both starters have a combined 6.30 ERA and Gilbert has been -red-hot lately, with just two earned runs in four of his last five starts. The Mariners have been struggling against lefties and both teams have top-6 bullpens, so I expect a low-scoring affair. Take the under 6.5 in this one.

Nikos Lagouretos's Pick: Under 6.5