The Boston Celtics drafted Nesmith with the 14th pick of the 2020 NBA Draft. He made his NBA debut on Christmas Day with three points over nine minutes in a loss to the Nets. The rookie appeared in 46 of Boston's 72 games, with one start. On Jan. 24, Nesmith posted 11 points, six rebounds and three triples in a home win over the Cavaliers. For an eight-game run starting in mid-February, Nesmith was a regular in the Celtic rotation. During that span, Nesmith averaged 5.3 points, 3.6 rebounds, 0.4 steals, 0.4 blocks and 1.0 three-pointers over 20.5 minutes per contest. Starting in mid-April, Nesmith appeared in each of Boston's last 15 games. On Apr. 30, the Vanderbilt product scored a career-high 16 points (7-9 FG, 2-3 3Pt) to go with six boards and one steal during a home win over the Spurs. Two days later, Nesmith again scored 16 points, plus drained four three-pointers, in a home loss to Portland. On May 12, Nesmith received his first career start and posted eight points, seven rebounds and two assists over 22 minutes at Cleveland. In college, Nesmith was known for his outside shooting. With Boston, Nesmith shot 37.0 percent from behind the arc, which ranked 12th amongst all rookies with 50+ attempts. Nesmith appeared in all five of Boston's playoff games versus Brooklyn. In the Celtics' Game 4 loss, Nesmith scored 11 points, hit three three-pointers and grabbed two boards.