The announcement was made on Monday.
Since retiring at the end of the 2015-16 season, Duncan had been spotted on occasions practicing with the Spurs players at the team's facility.
Duncan, 43, played 19 seasons with the Spurs and won five National Basketball Association championships playing for Popovich from 1997-2016.
Hardy has served as the head coach of the Spurs' Summer League team in Utah and at one point, appeared as the head coach of the Spurs' 2017 Spurs MGM Resorts Summer League team.
Lowry close to breakthrough British Open win
Holmes teed off in third place, six strokes behind Lowry, yet it was to prove anything but elementary for the man from Kentucky. Italian Francesco Molinari finished his year as Open champion with a fine 5-under-par 66 to be 3-under-par for the tournament.
Duncan is a 15-time NBA All-Star, was the Rookie of the Year in 1998, is a two-time NBA MVP and 15-time All-Defensive Team player.
The prevailing wisdom at the time was that Pop would continue coaching the Spurs until at least the 2020 Olympics, but there was no guarantee he would return until he signed a three-year contract extension.
Hiring Tim Duncan as the next head coach would certainly win the press conference in San Antonio, and we know that this organization loves to hire within to maintain their historically great culture.
Duncan and Spurs video coordinator Will Hardy will join Chip Engelland and Becky Hammon on the bench alongside Popovich. Will Hardy also has been elevated to assistant coach.