On January 17, 2018, the Tigers avoided arbitration with Castellanos, agreeing on a one-year contract worth $6.05 million. On August 13, Castellanos went 5-for-5 with 5 RBIs, for his first career five-hit game. He became the first Tigers player with five hits in a game since Ian Kinsler in 2015. With two singles, two doubles and a home run, he also became the first Tiger with 10 total bases in a game since Justin Upton in 2017. Castellanos earned American League Player of the Week honors for the week of August 13–19. In seven games, he batted .393 with an OBP of .485, and had two homers, three doubles, eight runs scored and ten RBI. The award was the first weekly honor for any Tiger player in 2018, and the first since J.D. Martinez won the award for the week ending July 17, 2017.
Castellanos hit a career-high .298 in the 2018 season, adding 23 home runs and 89 RBI. He finished among the AL leaders in hits (185, third), doubles (46, fourth) and multi-hit games (56, third). He also led all MLB hitters (60 or more plate appearances) in batting average against left-handers, at .381. He was named the 2018 Tiger of the Year by the Detroit Chapter of the Baseball Writers Association of America (BBWAA).