Monomoy Girl began her three-year-old campaign in the Grade II Rachel Alexandra Stakes at Fair Grounds Race Course on February 17, 2018 and started the 9/10 favorite ahead of Wonder Gadot (winner of the Demoiselle Stakes) and Classy Act. She broke poorly and hit the side of the starting gate, then settled at the rear of the field in the early stages. On the final turn, the filly moved up on the outside, took the lead in the straight and won by two and a half lengths from Classy Act despite ducking to the right a furlong out. After the race Brad Cox said, "She proved herself today... You can see she's still a little bit of a funny filly to deal with, but she's learning and we're lucky to have her. Obviously the Kentucky Oaks is the goal".
