The honored soldier, Army Pfc. Ross McGinnis, "gave all for his country," the president said somberly.
"No one outside this man's family can know the true weight of their loss. But in words spoken long ago, we are told how to measure the kind of devotion that Ross McGinnis showed on his last day: 'Greater love hath no man than this, that a man lay down his life for his friends."'
The president spoke in the East Room at a ceremony attended by Vice President Dick Cheney, prior recipients of the Medal of Honor, military leaders, McGinnis' parents, Tom and Romayne, and his two sisters, Becky and Katie. The four soldiers protected by McGinnis' actions were all in attendance.
McGinnis was in the gunner's hatch of a Humvee on Dec. 4, 2006, on a patrol in Iraq, when a grenade sailed past him and into the vehicle where the four other soldiers sat. He shouted a warning, then jumped on the grenade while it was lodged near the vehicle's radio.
When the grenade landed in the vehicle, Ian Newland, one of the four men McGinnis saved, recalled that McGinnis barricaded his body against the explosive. "I immediately went to cover my face and then the grenade exploded," Newland said.
"By that split-second decision, Private McGinnis lost his own life, and he saved his comrades," Mr. Bush said.
McGinnis grew up in the rural town of Knox, Pa., about 60 miles northeast of Pittsburgh.
He enlisted in the Army after some struggles in school. Friends and family say they were watching him transform into a man.
During his junior year, McGinnis, who struggled academically, enlisted in the army. The military demanded a high school diploma, and it proved to be just the motivation he needed to graduate - about a week after he earned his diploma, McGinnis was in the army, reports CBS News correspondent Priya David.
Mr. Bush called him a regular guy, a dependable friend with a big heart and a carefree spirit. He also had a robust sense of humor, as was known as the only one in boot camp who could make his drill sergeant laugh, the president said.
As a military aide read the formal citation of McGinnis' honor, Mr. Bush gave the late soldier's mom a smile. When the audience rose and offered an ovation, Mr. Bush gave Mrs. McGinnis a kiss on the cheek and shook her husband's hand.
"As Ross had told us previously, he said, if you're hurt, the army wants you to call home. If you're hurt bad enough, they'll call for you. And if you're killed they make a personal visit to your door," Ross's father, Tom, said. "When I saw two officers out there, I just - I practically went into shock. I was just shaking all over."
Though Ross's parents said part of them wishes he had jumped out of the vehicle, they both acknowledged that their son made the right decision.
"If he'd have jumped out, four men probably would have died," his father said. "We would have had our son, but he wouldn't have had himself. I mean, he would have hated himself for the rest of his life."
The president said it was a high privilege for him to present the Medal of Honor, recognizing valor beyond anything that duty could require.
"May the deep respect of our whole nation be a comfort to the family of this fallen soldier," Mr. Bush said. "May God always watch over the country he served, and keep us ever grateful for the life of Ross Andrew McGinnis."