The controversial criminal court found Mohamed Nazim guilty of smuggling dangerous weapons into the upmarket tourist destination with the aim of toppling President Abdulla Yameen.
Nazim's family said he had not received a fair trial following the verdict, which came out overnight Thursday and is likely to intensify fears of growing political repression in the honeymoon island nation.
The verdict came less than two weeks after the court convicted opposition leader Nasheed under anti-terror laws for ordering the arrest of a chief judge in 2012 when he was president and sentenced him to 13 years in jail.
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President Yameen sacked Nazim in January shortly after police raided his flat in the capital Male and seized documents as well as a pistol and ammunition.
He was also accused of engineering a fire at the main desalination plant serving the island capital capital Male that triggered a water crisis in December.
Nazim, a retired colonel from the Maldivian National Defence Force, was minister of defence and national security.
Nasheed's jailing has sparked street protests in the Maldives and widespread international criticism including from the UN Human Rights Council.
His Maldivian Democratic Party has said the criminal court judges were biased, a charge the government has denied.
Nasheed's lawyers said this week the authorities had prevented them from filing an appeal by a Sunday deadline.
The rising political unrest threatens the reputation of the atoll nation of 340,000 Sunni Muslims, whose pristine beaches and secluded coral islands have long been a huge draw for honeymooners and other tourists.