Nidal Hasan, 42, was convicted last week of the 2009 shooting at Fort Hood, in which 32 others were also injured.
The defendant had tried to admit his guilt but military law bans guilty pleas in death penalty cases.
The Virginia-born Muslim said he opened fire to protect Taliban insurgents from troops about to deploy to Afghanistan.
Nidal Hasan’s court-appointed lawyers have previously told the judge they suspect he is seeking execution in a bid for martyrdom.
The jury handed down its sentence after two hours of deliberations on Wednesday.
But it could be years, possibly decades, before Nidal Hasan is executed because of the long appeals process in the military justice system.
His execution must eventually be authorized by the president.
On Wednesday before the sentence was handed down, prosecutor Colonel Mike Mulligan urged jurors to opt for a rare military death penalty.
“He will not now and he will never be a martyr,” Col. Mike Mulligan said of Nidal Hasan.
“He is a criminal. He is a cold-blooded murderer.”
“This is not his gift to God. This is his debt to society. This is the cost of his murderous rampage.”
“He will never be a martyr because he has nothing to give,” Col. Mike Mulligan added.
“He will not be giving his life, we will be taking it.”
Nidal Hasan, who represented himself, declined to speak on his own behalf, saying only: “I have no closing statement.”
The 13-member jury had to come to a unanimous agreement in order to sentence Nidal Hasan to death, otherwise he would have faced life in prison.
The US military has not executed a service member since 1961. There are five inmates on the US military’s death row at Fort Leavenworth, Kansas, all at various stages of the appeals process.
Nidal Hasan opened fire at a medical facility on the Fort Hood base where soldiers were being evaluated before deploying overseas.
The trial heard he had prepared carefully for the attack, during which he fired 146 bullets.
The shooting spree ended when he was shot by a civilian police officer.
Nidal Hasan was paralyzed from the waist down from the wound and now uses a wheelchair.
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