A former governor of Puerto Rico, Wanda Vázquez, pleaded guilty on Wednesday to a misdemeanor campaign finance violation for accepting a promise of a political contribution from a Venezuelan banker in 2020.

The unusually lenient plea deal was backed by a top Trump Justice Department official and grudgingly accepted by a federal judge, who likened it to a slap on the wrist.

Ms. Vázquez, 65, pleaded guilty to accepting a promise of a donation greater than $2,000 but less than $25,000 from a foreign national in the Federal District Court in San Juan. That is a far less serious offense than the charges of felony conspiracy, federal programs bribery and honest services wire fraud brought against her in 2022, which carried up to 20 years in prison.

She will be sentenced on Oct. 15 to between six months and a year of probation, under an agreement between prosecutors and her defense lawyers.

Ms. Vázquez, a Republican who endorsed Mr. Trump and a member of Puerto Rico’s pro-statehood party, served as governor for less than two years, from 2019 to 2021, after a period of political tumult on the island.

She was arrested in 2022 after a grand jury indictment accused her of accepting bribes in exchange for replacing a top regulator on the island, who had been scrutinizing a bank owned by one of her campaign donors. The donor was Julio M. Herrera Velutini, a Venezuelan whose international bank, Bancrédito, was mired in regulatory problems related to suspicious banking transactions.