Jobu University After retiring as a player in 2018, in 2021 he became manager of
Jobu University's football team.
Kashima Antlers For the 2022 season, Iwamasa became the assistant manager of
Kashima Antlers and took charge of a number of games whilst new manager
René Weiler awaited entry to the country due to COVID-19 quarantine restrictions. In August 2022, it was announced that Iwamasa would be promoted to manager of Kashima Antlers following the departure of Weiler. The first game in the charge was 2-0 home win by
J1 League against
Avispa Fukuoka in 14 August 2022. But the in the nine games after. only one win against Vissel Kobe in
Emperor's Cup, the team was knocked out by second tier team
Ventforet Kofu after defeat by 1-0. Ending the season in fourth place, three points behind of the
AFC Champions League slots occupied by
Sanfrecce Hiroshima. The 2023 Season starts with an away victory over
Kyoto Sanga by 0-2. The first match at home was a defeat against current runners-up
Kawasaki Frontale by 1-2. Following this, the team collected mixed results in the Emperor's Cup. They were knocked out by the defending champions, Ventforet Kofu for a consecutive season after a 1-1 draw, the team lost 10-11 in the penalty shootout. Following the end of the campaign, Antlers won the last match of the competition against the relegated team
Yokohama FC.), ending the aspirations for qualification to AFC Tournaments after a 3-1 defeat to Vissel Kobe. On 5 December 2023 the board announced the Iwamasa's contract was not renewed for the 2024 season.
Hà Nội On 11 January 2024, Iwamasa was named as the manager of
V.League 1 club
Hà Nội. He lost his first
V.League 1 match, 2–0 at away to
Dong A Thanh Hoa on 18 February. He resigned his role as head coach on 8 July.
Hokkaido Consadole Sapporo On 12 December 2024, Iwamasa was announced as manager of newly relegated club
Hokkaido Consadole Sapporo for the
2025 J2 League season.{{Cite web|url=https://www.consadole-sapporo.jp/news/2024/12/11061/|title=岩政 大樹氏 監督就任のお知らせ ==Career statistics==