Jackson appeared in 79 games for the Phoenix Suns during his second year in the NBA. He started 29 of those contests, playing 25.2 minutes per game and averaging 11.5 points per contest. This marked the second-straight year Jackson averaged double figures in scoring. He excelled from three-point field range by shooting 32.4 percent. In fact, over the final 17 games of the season, Jackson hit over 40 percent of his attempts from three-point land. He canned a career-high five treys in an overtime win over New Orleans on Apr. 5, en route to a career-best 35 points. Jackson also flashed his versatility, managing 4.4 rebounds and 2.3 assists per contest. He notched two double-doubles, and averaged 8.0 rebounds per game during the month of April. Jackson also posted eight assists against the Clippers on Dec. 10. He'd go on to finish fifth in scoring and sixth in rebounding for the Suns.

Following one season at Kansas, Jackson was selected with the fourth overall pick in the 2017 NBA Draft. He made his NBA debut in the first game of a season against Portland on Oct. 18. Jackson played 32 minutes as a starter, posting 11 points and two rebounds. He ended up appearing in 77 games with 35 starts. As a reserve, he averaged 11.4 points, 4.5 rebounds and 1.3 assists. As a starter, Jackson averaged 15.3 points, 4.7 rebounds, 1.9 assists and 1.1 steals. He set single-game highs of 36 points, 12 rebounds, five assists, four steals and four blocks. March 17 marked his best performance of the season, which featured 36 points, six rebounds, four assists, two steals and a block over 36 minutes in a loss to Golden State. Jackson's best stretch of the season came in his final seven games. From March 24 through April 6, he averaged 22.6 points, 5.1 rebounds, 3.0 assists and 2.1 steals over 36.3 minutes. The Suns finished the season with a 21-61 record and did not make the playoffs.