In July 2015, Stepheson joined the Dallas Mavericks for the 2015 NBA Summer League. On October 7, 2015, he signed with the Memphis Grizzlies. However, he was waived on October 24 after appearing in three preseason games. On October 31, he was acquired by the Iowa Energy of the NBA Development League as an affiliate player of the Grizzlies. On November 14, he made his debut with Iowa in a 98–95 win over the Sioux Falls Skyforce, recording 13 points, 17 rebounds, two assists and one steal in 37 minutes. On January 29, 2016, he was named in the East All-Star team for the 2016 NBA D-League All-Star Game after averaging 15.7 points, 13.3 rebounds, 1.3 blocks and 34.3 minutes in 26 games.