SANTA ANA, Calif.—As an interpreter, Ippei Mizuhara was supposed to bridge the gap between baseball star Shohei Ohtani and his English-speaking teammates and fans as the duo traveled from Southern California to ballparks across the United States.

Instead, Mr. Mizuhara exploited the Japanese-English language barrier to isolate Mr. Ohtani and profit, in the truest sense, from his proximity to the two-way player ’s power. On Tuesday, the ex-interpreter pleaded guilty in federal court in Santa Ana, California, to bank and tax fraud for stealing nearly $17 million from the unsuspecting athlete’s Arizona bank account.

He spent the money to cover his growing gambling bets and debts with an illegal bookmaker, plus $325,000 worth of baseball cards and, to the shock of prosecutors, his own medical bills.

“In fact, after we announced the charges, we only discovered more fraud in this case,” said Martin Estrada, U.S. Attorney for the Central District of California. “We discovered Mr. Mizuhara had victimized Mr. Ohtani to the extent that he wouldn’t even pay for dental. He stole money from Mr. Ohtani to pay for his own dental expenses.”

The case involved arguably the world’s most famous baseball player and the sport’s most valuable voice. Despite the international media frenzy, Tuesday’s 45-minute proceeding was fairly mundane: Mr. Ohtani was known as “Victim A” inside the courtroom and the ex-interpreter only spoke to acknowledge his guilt.

“I worked for Victim A and had access to his bank account and had fallen into major gambling debt,” Mr. Mizuhara told the judge. “I went ahead and wired money … with his bank account.”

He and his attorney declined to comment after the hearing.

Inside baseball, Mr. Mizuhara stood by Mr. Ohtani’s side for many of the Japanese sensation’s career highlights, from serving as his catcher during the Home Run Derby at the 2021 All-Star Game, to being there for his two American League MVP wins and his record-shattering $700 million, 10-year deal with the Los Angeles Dodgers.

Off the field, Mr. Mizuhara became Mr. Ohtani’s friend and confidant. He famously resigned from the Los Angeles Angels during the 2021 MLB lockout so he could keep speaking to Mr. Ohtani—he was rehired after a deal was struck—and their wives reportedly socialized.

But Mr. Mizuhara gambled it all away, betting tens of millions of dollars that weren’t his to wager on international soccer, the NBA, the NFL, and college football—though prosecutors said he never bet on baseball.

Mr. Estrada, the U.S. attorney, said Mr. Ohtani was particularly vulnerable, despite his fame.

“Mr. Ohtani is an immigrant who came to this country, is not familiar with the ways of this country and therefore was easily prey to someone who was more familiar with our financial systems,” Mr. Estrada said during a news conference in downtown Los Angeles after the hearing.

Federal prosecutors said Mr. Mizuhara’s scheme began in 2021 when he switched the bank account’s contact information from Mr. Ohtani’s to his own, meaning any communication from the financial institution would be sent directly to him without Mr. Ohtani knowing.

Mr. Mizuhara capitalized on the language barrier to keep Mr. Ohtani’s financial advisers from understanding their client, and at times, Mr. Mizuhara even impersonated the player to the bank to prolong the fraud.

The ploy allowed Mr. Mizuhara to plunder just under $17 million from the account—which he’d helped Mr. Ohtani set up in Phoenix in 2018 to deposit his paychecks—from 2021 until earlier this year.

Mr. Mizuhara’s winning wagers totaled over $142 million, which he deposited in his own bank account and not Mr. Ohtani’s. But his losing bets were around $183 million, a net loss of nearly $41 million.

Tuesday’s guilty plea was anticipated after Mr. Mizuhara agreed to a deal with the U.S. Attorney’s Office last month. He pleaded guilty to one count of bank fraud, which carries up to 30 years in federal prison, and one count of subscribing to a false tax return, which could add a maximum of three years of incarceration.

A sentencing hearing is scheduled for October. Mr. Mizuhara also could be on the hook for restitution to Mr. Ohtani that could total nearly $17 million, as well as more than $1 million to the IRS. And as a legal permanent resident who has a green card, he might be deported to Japan.

The investigation into Mr. Mizuhara stemmed from a broader probe of illegal sports bookmaking organizations in Southern California and the laundering of proceeds through casinos in Las Vegas. Overall, authorities have netted a dozen defendants.

There was no evidence Mr. Ohtani was involved in or aware of Mr. Mizuhara’s gambling, and the player cooperated with investigators. He expressed relief after Tuesday’s hearing and, in a statement, thanked his team, family, and the Dodgers organization “who showed endless support throughout this process.”

“It’s time to close this chapter, move on and continue to focus on playing and winning ballgames,” he said.

The Los Angeles Times and ESPN broke the news of the prosecution in late March, prompting the Dodgers to fire the interpreter and MLB to open its own investigation.

While prosecutors said Mr. Mizuhara never bet on baseball, MLB rules prohibit players and team employees from wagering with illegal or offshore bookmakers.

MLB also bans betting on baseball, even legally. On Tuesday—hours before Mr. Mizuhara’s hearing—the organization banned San Diego Padres infielder Tucupita Marcano from baseball for life in the wake of another gambling scandal.

MLB said Mr. Marcano placed 387 baseball bets totaling more than $150,000 in October 2022 and from last July through November with a legal sportsbook. He became the first active player in a century banned for life because of gambling. Four others were suspended Tuesday.

By Stefanie Dazio and Amy Taxin