BANGKOK — An outspoken member of Malaysia’s Parliament was sentenced on Monday to 18 months in prison for publicly disclosing classified information from an official audit into a scandal-plagued government investment fund.

A lower court ruled that the lawmaker, Rafizi Ramli, was guilty of violating the Official Secrets Act by possessing and publicizing information from the document. Mr. Rafizi, who has served in Parliament since 2013, could also lose his seat and be barred from running for office for five years.

Rights advocates said the prosecution and conviction of a sitting member of Parliament for speaking publicly was unprecedented and was aimed at silencing one of the government’s most vocal critics.

“The 18 months’ imprisonment sentence can only be described as harsh and excessive, all the more so as Rafizi was merely performing his role as an elected representative,” Lawyers for Liberty, a Malaysian human rights organization, said in a statement. “The conviction and sentence will create a dangerous chill on free speech and result in a more repressive, opaque and unaccountable government.”

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.

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.

Cynthia Gabriel, director of the Center to Combat Corruption & Cronyism, based in Malaysia, questioned the purpose of having an audit if the findings were to be kept secret.

“The Official Secrets Act is being used to hide corruption,” she said. “We need freedom of information laws to help the public monitor and bring to account powerful politicians and businesses.”

The prime minister’s office defended the prosecution of Mr. Rafizi by saying that he broke the law to make a political point and wanted to become a “political martyr.”

“He tried a cheap stunt for personal political gain, but he knowingly committed a serious crime in doing so,” said Abdul Rahman Dahlan, a minister in the office. “It is right that he pays the price — and he has only himself to blame.”

Opponents of the prime minister plan to hold a rally on Saturday in Kuala Lumpur, the capital. A similar event last year drew as many as 100,000 people, most wearing yellow T-shirts with the slogan, “Bersih,” or “clean” in Malay, despite a government ban on the garments. A court later upheld the prohibition on the grounds that the shirts posed a threat to national security.

Maria Chin Abdullah, a leader of the Bersih movement, said the Official Secrets Act gives the prime minister extraordinary power to suppress potentially damaging information.

“The act vests vast powers in the hands of the executive to conceal key information from public access and to decide on what constitutes ‘official secrets,’ which cannot be challenged in court on any grounds,” she said.

Phil Robertson, deputy director of the Asia division of Human Rights Watch, said the conviction went further than the government’s previous steps to block criticism.

“This prosecution really is unprecedented because it involves a sitting MP, and the content is the Auditor General’s annual report, which prior to this year has regularly been released to the public after being introduced in Parliament,” he said.