After going undrafted in the 2016 NBA draft, Forbes joined the San Antonio Spurs for the 2016 NBA Summer League. Heading into the Summer League, Spurs' head coach Gregg Popovich told Forbes "to shoot it" after he made 77-of-100 three-pointers at one of his NBA pre-draft workouts. On July 14, 2016, he signed with the Spurs. Forbes secured an opening-night roster spot after impressing the Spurs during training camp and preseason. Forbes played sparingly for the Spurs throughout his rookie season, spending most of his time in the NBA Development League on assignment with the Austin Spurs. On April 7, 2017, with Tony Parker, Manu Ginóbili and Kawhi Leonard not playing, Forbes scored a career-high 27 points to lead the Spurs to a 102–89 win over the Dallas Mavericks. Forbes, who had 64 points for the season with a game high of eight coming in, went 4 of 9 from 3-point range and also had career highs in minutes (29) and assists (six).