The court martial in the eastern city of Wonju condemned the 22-year-old sergeant, surnamed Lim, to death for committing what it called "premeditated and cruel" killings, Yonhap news agency said.
Lim remained silent, keeping his head lowered throughout the sentencing.
The case will now go to an appeals court and then to the Supreme Court, an automatic procedure in South Korea in the case of death sentences.
In his pre-sentence statement on January 23, Lim said he was deeply remorseful for his actions.
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"I feel deeply sorry for the victims and their relatives," Lim said in the statement.
"I am overcome with remorse... How happy would I be, had it been merely a dream and had it been possible to return to the past?"
In June last year Lim threw a grenade and opened fire on members of his unit, killing five and wounding seven, near a frontline guard post.
Lim, who was listed as a soldier requiring special observation, told investigators he carried out the attack in revenge for constant mockery and harassment against him.
Barrack-room bullying has long tainted South Korea's military service. It has been blamed for numerous suicides and incidents where conscripts have turned their weapons on their comrades.
Conscripts, most of them in their early twenties, account for the lion's share of the military's 690,000 active personnel.
In 2011 a 19-year-old marine conscript killed four colleagues on a western island near the border. Eight soldiers were killed in 2005 when a conscript threw a grenade and sprayed bullets over the sleeping men at a frontline guard post.