"Major League Baseball has been gathering information since we learned about the allegations involving Shohei Ohtani and Ippei Mizuhari from the news media," an MLB spokesperson said.
"Earlier today, our Department of Investigations (DOI) began their formal process investigating the matter."
Mizuhara was fired by the team on Wednesday after Ohtani's lawyers said he had engaged in "massive theft."
U.S. media reported that Mizuhara allegedly used Ohtani's money to pay off gambling debts to a Southern California bookmaker, who is under federal investigation.
Two-time AL MVP Ohtani inked a record 10-year, $700 million contract with the team in the offseason.
The allegations surfaced during the Seoul Series, the first scheduled MLB games in South Korea, with games between the Dodgers and the San Diego Padres.
Ohtani's lawyers did not respond to a request for comment and Reuters has been unable to reach Mizuhara for comment.
