According to cllct's Darren Rovell, the suit was filed on Wednesday evening in Florida -- just hours after Ken Goldin revealed his auction house will put the ball up for sale at the end of this week with a starting bid of $500,000.

Rovell reported the plaintiff is an 18-year-old named Max Matus ... who's convinced he's the rightful owner of the baseball. Matus alleges in his suit, according to Rovell, that he had full control of the ball before Chris Belanski wrongfully stole it from him during a scrum.

Rovell stated Matus is suing Belanski and Goldin -- seeking, among other things, to stop their impending sale of the memento.

But, Goldin said in a message to TMZ Sports on Thursday there are no plans as of now to pause things ... and bidding will still go down on Friday at 9 AM PT.

"Having reviewed the allegations and images included in the lawsuit," Goldin said, "and publicly available video from the game, Goldin plans to go live with the auction of the Ohtani 50/50 ball as planned."

"We note that while Goldin has also been named as a party in the case," he continued, "there are no allegations of wrongdoing by the company."

"Otherwise, we will not comment on the pending litigation."

Ohtani whacked the ball over the LoanDepot Park fence during the seventh inning of the Dodgers' tilt with the Marlins on Sept. 19 -- becoming the first-ever player in MLB history to record a season with 50 home runs and 50 stolen bases.