After going undrafted in the 2014 NBA draft, Cotton signed with the San Antonio Spurs on July 7, 2014. He played in six Summer League games and five preseason games for the Spurs, before being waived on October 23, 2014. He subsequently joined the Spurs' NBA Development League affiliate team, the Austin Spurs, and appeared in 34 games (all starts), averaging 22.4 points (third in the D-League), 4.7 rebounds, 4.6 assists and 1.3 steals in 40.5 minutes (first in the D-League) per contest. Cotton scored in double figures on 31 occasions and recorded 20-plus points in 20 games. He also registered 40-or-more points twice, including a season-high 43 points, eight rebounds, nine assists and two steals against the Reno Bighorns on February 19, 2015. He was named D-League Performer of the Week on February 23 after averaging 30.0 points, 5.7 rebounds, 5.3 assists and 1.3 steals for the week. Earlier that month, he was named to the Western Conference All-Star team for the NBA D-League All-Star Game. At the season's end, he was named All-NBA D-League Second Team and NBA D-League All-Rookie First Team.