On April 23, 2014, he drove in five runs to avoid a sweep against the Houston Astros. He hit a two-run home run in the seventh inning to cut the Astros lead to 3–2, and hit a walk-off three-run shot in the bottom of the ninth, allowing the Mariners to win 5–3. The first walk-off hit of his career, his performance earned him an April Co-Player of the Week Award, shared with José Abreu of the White Sox. On June 2, Seager went 4–4 with a double, two triples and a three-run home run against the New York Yankees. The first Mariner to accomplish the feat, he became the first MLB player with two triples and at least one double and homer in a game since Hal Breeden for the Montreal Expos in 1973. On June 15, he went 4–4 with two singles and two doubles and three RBI. On July 7, he was named to his first All-Star team as an injury replacement for Toronto Blue Jays player Edwin Encarnación. He finished 2014 with a .268 batting average, 25 home runs and 96 RBI in 159 games.
On December 2, 2014, the Mariners finalized a 7-year, $100 million contract extension with Seager. In 2015, he hit .266 with 26 home runs and 74 RBI in 161 games.