Seager received a non-roster invite to the Dodgers major league spring training in 2015. MLB.com ranked him the 7th-best prospect in baseball going into the 2015 season, and Baseball America named him the #5 prospect in 2015. The Dodgers assigned Seager to their new AA affiliate, the Tulsa Drillers of the Texas League, to start the 2015 season. In 20 games with the Drillers, Seager hit .375 with five homers and 15 RBI. Manager Razor Shines said of him, "I'm running out of words to describe this kid. He's phenomenal."
On May 1, 2015, Seager was promoted to the Oklahoma City Dodgers of the Class AAA Pacific Coast League (PCL). In a game on May 28 against the Salt Lake Bees, Seager had six hits in six at-bats, including a home run, and six RBI. He was the second player in the history of the Oklahoma City franchise to record six hits in one game. He was selected to the mid-season PCL All-Star team. In 104 games for Oklahoma City, Seager hit .276 with 13 homers and 59 RBI. Baseball America selected him to their Minor League All-Star team, and named him a AAA All-Star and the AAA Player of the Year.
On September 3, 2015, the Dodgers called Seager up to the majors, and he made his debut that night as the starting shortstop against the San Diego Padres. He had two hits in four at-bats with two RBI in his debut, with his first MLB hit being a double to right field off of Colin Rea of the Padres.
On September 12, 2015, against the Arizona Diamondbacks, Seager was 4-for-4 with his first MLB home run (off Josh Collmenter), a walk and a stolen base, making him the third-youngest player in history to accomplish that feat (after Ken Griffey, Jr. and Orlando Cepeda). On September 21, Seager passed Bill Russell by reaching base safely in his first 16 major league starts, a new Dodger record. He hit .337 in 27 games with the Dodgers, with four homers and 17 RBI, supplanting Jimmy Rollins as the Dodgers starting shortstop down the stretch. He was the starting shortstop for the Dodgers in the first game of the 2015 National League Division Series, making him the youngest position player to start a postseason game in franchise history.