Millions of Chinese were killed during the brutal fight against Imperial Japanese forces, whose record of massacre, rape and looting still strains relations between Beijing and Tokyo.
Brighton's academy apologised on Saturday after the post - which showed Mitoma posing with a young player and a mocked-up football card featuring Imperial Army officer Hiroo Onoda - caused an outcry.
At noon on Monday, the topic "Premier League team apologises to Chinese fans" was the top-trending item on Chinese social media platform Weibo and had been viewed more than 15 million times.
"There are actually Premier League teams that support fascists," read the top-liked comment under a post by state-backed tabloid The Global Times.
Others called for Brighton to terminate Japanese international Mitoma's contract.
"It is hoped Chinese football fans won't let him off the hook. We must make him understand our resolve, learn a lesson, or else there will be more Kaoru Mitomas who raise their heads," another commenter said.
It is not clear how the image of Onoda, an officer who refused to believe World War II was over and waged a guerrilla campaign in the Philippine jungle for three decades, came to be made into a football card.
Brighton said the picture of Mitoma holding it was taken at an event leading up to the Premier League Christmas Truce Tournament, which sees under-12 players from eight English clubs compete against European ones.
The tournament is named for an impromptu December 25th truce in World War I, when British and German soldiers met in no-man's land, with some playing football together.
The Premier League describes the competition as an event where youth players can gain "understanding of the historical events that shaped our world".
Brighton apologised for "offence caused in China".
"We hugely value our fans in China and had no intention of causing any offence," the club's youth academy said in the post on X on Saturday.
The statement was reposted by the club's main account.
Mitoma, who has been capped 29 times by the Japanese men's national team, has not commented publicly.
Relations between Beijing and Tokyo are particularly testy after Japanese Prime Minister Sanae Takaichi suggested Tokyo could intervene militarily in any attack on Taiwan, which China claims as its territory.
AFP has contacted Brighton for comment.