Seattle Mariners vs Kansas City Royals Prediction, Odds and Picks for Today, 09/18/25

Thursday’s matchup between the Kansas City Royals and Seattle Mariners should be an interesting game from both a betting and viewership perspective. The Mariners are a half-game back of the Astros for first place in the AL West and the Royals are looking to play spoiler with their .500 record. Seattle has had a successful season already by their standards, but mid-season acquisitions of power bats indicate they are geared up for a postseason run as well. Luis Castillo is set to start this game for the Mariners, and the veteran right-hander enters this matchup with a 3.76 ERA. His last 2 starts were very strong following 3 shakier starts, and despite his rebound performances I still favor the Royals starter in this spot.

Stephen Kolek was acquired by the Royals from the Padres at the trade deadline and the right-hander has made 3 starts with his new team since the trade. Those 3 outings have resulted in 19.2 innings of 4-run ball with just 2 walks issued in that span. While his career-long track record is not as strong as Castillo’s, his current form is something that is exciting Royals fans. At current pricing I side with Kansas City in their home park.

Kansas City Royals vs Seattle Mariners prediction: Royals ML (+100) available at time of publishing. Playable at that price.