He made his debut for the Celtics in their season opener on October 26 against the Brooklyn Nets.
On November 18, he set a then career-high with 27 points and helped the Celtics win their 15th straight game with a 110–99 victory over the Atlanta Hawks.
On December 13 he had a 26-point effort against the Denver Nuggets.
On April 6, 2018, he set a then career-high with 32 points in a 111–104 win over the Chicago Bulls.
In Game 2 of the Celtics' first-round playoff series against the Milwaukee Bucks, Brown had a then playoff career-high 30 points in helping Boston take a 2–0 series lead with a 120–106 win.