On June 25, 2015, Booker was selected with the thirteenth overall pick by the Phoenix Suns in the 2015 NBA draft. On July 13, he signed his rookie scale contract with the Suns, and in seven subsequent Summer League games, he averaged 15.3 points, 4.9 rebounds and 1.7 assists per game. He went on to make his NBA debut two days before his 19th birthday, in the Suns' season opener against the Dallas Mavericks. He became the first player in NBA history to debut at 18 years old after playing at least one year of college basketball. In 21 minutes of action against the Mavericks, he scored 14 points on 6-of-7 shooting in a 111–95 loss. While he began the season as a backup shooting guard throughout the early portion of the season, including starts on November 23 and December 21, a late December injury to starting guard Eric Bledsoe on December 26 led to Booker's promotion to the starting line-up for the rest of the season. Before his teammate's injury, Booker was producing averages of 5.4 points, 0.9 rebounds, and 0.8 assists in 14.1 minutes of play. His production then increased throughout the second half of the season.