On Friday, a federal appeals court in Boston ordered that three of Tsarnaev's convictions have been reversed, his death penalty scrapped and a new penalty trial ordered.
"It's ridiculous. Think of that. It's ridiculous," Trump said in Florida, calling Tsarnaev an "animal that killed so many people."
Earlier on Friday, US Court of Appeals overturned death penalty for Tsarnaev, ordering that a lower court must hold a new trial, with new jury empaneled, to "preside over a new trial strictly limited to what penalty Dzhokhar should get on the death-eligible counts".
The Boston Marathon bombing on 15 April, 2013, killed three people and injured more than 260 of others. Dzhokhar and his older brother Tamerlan were found guilty of the attack. Tamerlan was killed by police while on the run on 19 April of the same year, while Dzhokhar was arrested the following day and sentenced to death by lethal injection in May 2015.