An Urawa fan unveiled a 'Japanese Only' banner during the game, with officials eventually removing the seemingly racist remark during the second half of Sagan Tosu's 1-0 win.
The backlash from the banner was forthright, with Urawa defender Tomoaki Makino tweeting his disapproval of the message.
The club officially responded with a statement on Tuesday.
"It is extremely regrettable that such an incident occurred during Urawa Reds' first home match of the season despite our efforts to reinforce the SPORTS FOR PEACE! Project from this season," the statement read.
"We extend our deepest apologies to all who love and support Urawa Reds and all who may have been offended by it. We will reflect deeply on this incident and do our utmost to act more promptly in the future.
"This club categorically holds all discriminatory remarks and actions to be completely unacceptable. We are convinced that the most effective way to defeat discrimination is through the hearts and minds of each and every individual."
Security at the Reds' Saitama Stadium also reported that discriminatory remarks were heard from certain sections of the crowd.
The club is questioning the person who deployed the banner, while the J.League have pledged to investigate the incident as well.
