The tweet was posted late Saturday after Urawa were beaten by Gamba Osaka 3-1 before the J-League championship final, with Gamba’s striker Patric scoring the decisive goal just before the final whistle.
The posting “Drop dead, black” was made by an anonymous Twitter user whose handle is @takoj2000.
The person lists himself as an Urawa supporter, using photos of the club’s red-uniformed footballers for Tweet icon and banner.
“When celebrating the victory with teammates, I found the very daunting, horrible racist SNS posting on my mobile… I never thought I would be treated like this in this country,” Patric wrote later on his micro-blog.
Urawa reacted quickly, announcing that the club would not tolerate any discriminatory actions by its supporters.
“We have not identified who did the posting for what purposes, but Urawa Red Diamonds will never tolerate any discrimination,” the club said in a statement.
This was not the first time that Urawa were hit by a racist scandal.
Last year they were ordered to play a home game in front of an empty stadium in an unprecedented punishment after fans hoisted a racist “Japanese only” banner.
J-League imposed the heavy punishment after an investigation indicated severe damage to the image of Japanese football.
Urawa, the 2007 Asian champions and Japan’s best-supported club who have a large and passionate fan base, were also punished in the past for racist chants from their supporters.