Mr. Rafizi, a member of the People’s Justice Party, has been a leading critic of Prime Minister Najib Razak, who is accused of receiving $1 billion from 1 Malaysia Development Berhad, or 1MDB, a government investment fund that Mr. Najib established and oversaw. Mr. Najib has said that he never received any money from the fund or took anything for personal gain.

The United States Justice Department says that more than $3 billion is missing from the fund and that at least $731 million of it was deposited into the personal bank account of the prime minister, identified as “Malaysian Official 1.” The Justice Department filed suit in federal court in California in July to recover more than $1 billion in assets that it said were acquired by Mr. Najib’s stepson and close associates in the United States with money stolen from the fund, including high-end real estate and expensive artwork.

The prime minister has held on to power by firing critics within his own party, blocking investigations and suppressing dissent. No one in Malaysia has been prosecuted over the missing money.

The government conducted an audit of the investment fund, which it then classified as secret under the Official Secrets Act. Mr. Rafizi’s conviction was based on comments he made at a news conference in March in which he discussed a page of the audit that dealt with the fund’s failure to make payments.

Image Rafizi Ramli, a member of the People’s Justice Party. Credit... Associated Press

Around the time of his sentencing, Mr. Rafizi posted on Twitter: “I am not shocked, sad, angry, afraid or anything. No such feelings. Just another day. Been like this. What doesn’t kill u makes u stronger.” He did not respond to requests for comment, but associates said they expected him to appeal.