The decision was issued more than six months after arguments were heard in the case. Tsarnaev's brother died later that night after a gunfight with police, which ended when Dzhokhar ran him over with a stolen vehicle.
A Boston jury convicted Tsarnaev on all 30 charges he faced in 2015, including conspiracy and use of a weapon of mass destruction.
Mr. Tsarnaev and his older brother set off a pair of homemade pressure-cooker bombs near the finish line of the world-renowned race, tearing through the packed crowd and causing many people to lose legs.
Dic Donohue, a retired transit police sergeant who was critically wounded as law enforcement officers pursued the Tsarnaev brothers, and his wife Kim Donohue, have adopted an attitude of not allowing the attack to overtake their lives, regardless of what happens to Dzhokhar Tsarnaev.
Tsarnaev is now being held at a supermax prison in Florence, Colorado after being convicted on 30 charges, including conspiracy and use of a weapon of mass destruction.
Justice Thompson said the pervasive news coverage of the bombings and their aftermath featured "bone-chilling" photos and videos of Mr. Tsarnaev, now 27, and his brother carrying backpacks at the marathon and of those injured and killed near its finish line.
The higher court noted that the judge who presided over Tsarnaev's trial had rejected the defense team's request for a more distant trial venue where prospective jurors might be less likely to be prejudiced against the Chechen immigrant.
They pointed to social media posts from two jurors suggesting they harbored strong opinions even before the 2015 trial started. One of the posts had called Dzhokhar a "piece of garbage".
Tsarnaev admitted at his sentencing that he was "guilty of this attack, along with my brother". Attorney Daniel Habib claimed the city's anger over the attack "filtered" through the jury and the massive media coverage of the event meant no juror could be impartial.
As a result, Tsarnaev could again be sentenced to death, but the government must first decide whether it wants to pursue capital punishment.
"Despite a diligent effort, the judge here did not meet the standard set", Thompson stated.
US President Donald Trump weighed in on the ruling during an address to supporters on the tarmac of Tampa International Airport.
"I see in Boston, where you have the animal that killed so many people during the Boston Marathon", Trump said.