Qasim Ibrahim, who heads the Jumhooree Party (JP) and lost his bid for president in 2013, was accused of leading a failed bid in March to impeach Speaker Abdulla Maseeh, a close ally of Yameen.
He was convicted today of bribing lawmakers to impeach the speaker and given 38 months in jail. The guilty verdict also disqualifies Ibrahim, 65, from holding a seat in parliament.
The JP is one of four parties in the archipelago's opposition coalition that wants to defeat Yameen at elections next year and sees the impeachment of the speaker as a first step to taking control of parliament to ensure free and fair polls.
"The sentencing of MP Qasim Ibrahim once again confirms the lack of transparency and independence of Maldives' judiciary, and the breakdown of the entire criminal justice system," it said in a statement.
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"Like all other leaders of the opposition now sentenced to jail, the JP leader has been punished for challenging President Yameen's tyranny."
Ibrahim had collapsed in court yesterday during hearings and was rushed to hospital, where he is under treatment for a heart condition.
The verdict came a day after chaotic scenes at the national parliament where lawmakers flung eggs at each other while troops in plain clothing provided a ring of protection for the speaker inside the parliament chamber.
Since coming to power, President Yameen has led a crackdown on political dissent in the Indian Ocean nation of 340,000, raising fears over the country's stability and denting its image as an upmarket honeymoon destination.
Former leader Nasheed became the country's first democratically elected president in 2008, but was narrowly defeated by Yameen in a controversial 2013 election run-off.
He has lived in exile after Maldives authorities gave him leave to travel to London for medical treatment.