Iwamasa started his professional career at Kashima Antlers where he later became one of the club's longest serving players.

In 2014, Iwamasa moved to Thailand to join BEC Tero Sasana which he helped the club to win the 2014 Thai League Cup where he scored a goal in the final that ended in a 2–0 victory.

Iwamasa was the captain of the Japan team that won the 2003 Summer Universiade held in Daegu, South Korea, where he scored a goal in the final of the tournament.

He was also one of the final 23 Japan national football players participating in 2010 FIFA World Cup in South Africa although did not play any minute during the tournament.

Ending the season in fourth place, three points behind of the AFC Champions League slots occupied by Sanfrecce Hiroshima.

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.

On 12 December 2024, Iwamasa was announce official appointment manager of J2 relegated club, Hokkaido Consadole Sapporo from 2025 season.