Marcus Osmond Smart (born March 6, 1994) is an American professional basketball player for the Memphis Grizzlies of the National Basketball Association (NBA). He played college basketball for the Oklahoma State Cowboys.

Smart was drafted with the sixth overall pick in the 2014 NBA draft by the Boston Celtics. He initially came off the bench for the Celtics before starting games regularly in 2018 and becoming the starting point guard in 2021. He was named the NBA Defensive Player of the Year for the 2021–22 season, becoming only the sixth guard in NBA history to win the award. Smart has also been named to the NBA All-Defensive First Team three times and is a three-time NBA Hustle Award winner, winning it in 2019, 2022 and 2023. He helped the Celtics reach the NBA Finals in 2022.

In 2023, Smart was traded to the Grizzlies.