NEW DELHI — Rahul Gandhi, the leader of the main party opposing Prime Minister Narendra Modi of India, was convicted of defamation and sentenced to prison on Thursday, the latest blow to the beleaguered opposition party just a year before

national elections.

Mr. Gandhi was immediately granted 30 days’ bail. His party said he would appeal the conviction.

A court in Gujarat, Mr. Modi’s home state, sentenced Mr. Gandhi to two years in prison. He was convicted in connection with a 2019 speech in which he linked the prime minister’s family name to that of two Indian fugitives, Nirav Modi and Lalit Modi, accused of swindling millions of dollars.

“How come all the thieves have Modi as the common name?” Mr. Gandhi said while campaigning during the 2019 elections.

Purnesh Modi, a local legislator from the prime minister’s governing Bharatiya Janata Party in Gujarat, had filed the defamation suit against Mr. Gandhi, arguing that he found the comment offensive and hurtful.

Mr. Gandhi has been trying to revive the political fortunes of his once-mighty Indian National Congress Party, which has been under fire lately from Mr. Modi’s party after Mr. Gandhi questioned India’s democratic standing on a recent visit to London. Mr. Modi’s party called his remarks “anti-national.”

“The coward, dictatorial government of BJP is restless because Rahul Gandhi and the opposition are bringing their black deeds to light,” Mallikarjun Kharge, the president of the Congress party, said in a response on Twitter, referring to Mr. Modi’s party.

Mr. Gandhi’s party has said it will appeal his conviction to higher courts.