Ryan Lee Mountcastle (born February 18, 1997) is an American professional baseball infielder in the Baltimore Orioles organization.
Mountcastle attended Hagerty High School in Oviedo, Florida. He was drafted by the Baltimore Orioles in the first round of the 2015 MLB draft. He signed with the Orioles and was assigned to the GCL Orioles to begin his professional career. After batting .313 with three home runs and 14 RBIs in 43 games in the GCL, he was promoted to the Aberdeen IronBirds, where batted .212 in ten games to finish the season. Mountcastle spent 2016 with the Delmarva Shorebirds where he posted a .281 average with ten home runs and 51 RBIs along with a .745 OPS in 115 games. In 2017, he spent time between the Frederick Keys and the Bowie BaySox, batting a combined .287 along with a career high 18 home runs and 62 RBIs in 127 games between both teams. After the season, he played in the Arizona Fall League and played in the Fall Stars Game.
MLB.com ranked Mountcastle as Baltimore's third best prospect going into the 2018 season. He spent 2018 with Bowie, slashing .297/.341/.464 with 13 home runs and 59 RBIs in 102 games. Mountcastle spent 2019 with the Norfolk Tides, and switched to first base. After slashing .312/.344/.527 with 25 home runs and 83 RBIs over 127 games, he was named the International League Most Valuable Player.
Mountcastle was added to the Orioles 40–man roster following the 2019 season.