The two juveniles were convicted by the Juvenile Court in the capital Male yesterday over a fatal gang stabbing incident in February.
Both the accused, who have now reached 18, reportedly deny the charge.
"The Maldives authorities are flouting international law - anyone convicted of a crime committed when they were under 18 is exempt from the death penalty," said Polly Truscott, UK-based Amnesty International's Deputy Asia-Pacific Director.
"The authorities must immediately reverse these death sentences, and the prosecution must not try to uphold the death sentences in any appeals," said Truscott.
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"The sentences of all other prisoners on death row should be commuted, and an official moratorium on executions established, towards abolishing the death penalty. The Maldives is entering new and dangerous territory - imposing death sentences for crimes allegedly committed by children is alarming," she said.
The two teenagers have 90 days to appeal the death sentences at the High Court.
Another teenager was apparently acquitted due to a lack of evidence, while murder charges were filed against several others in connection with the attack.
"Amnesty International opposes the death penalty in all cases without exception. There is no convincing evidence that the death penalty works as a special deterrent against crime," said Truscott.