President Obama presented Hagel, who twice was awarded the Purple Heart for injuries he received in the Vietnam war, as his nominee to run the Pentagon during an announcement at the White House.
Hagel, a Republican who represented Nebraska in the U.S. Senate, "is the leader our troops deserve," Obama said. "He is an American patriot."
Obama nominated Hagel to succeed Leon Panetta.
Hagel "recognizes American leadership is indispensable in dangerous world," Obama said, and he knows America must "stand strongest" with its allies and friends.
In Hagel, Obama said, men and women in uniform would see a "combat vet of character and strength ... They see one of their own."
"Chuck knows war is not an abstraction," Obama said. "He understands sending young Americas to fight ... and bleed ... is something we do only when necessary."
Hagel said he was grateful for the opportunity to strengthen the country's military posture, its alliances and "to help build better world for all mankind. I will do my best."
Hagel's nomination is opposed by members of both the Republican and Democratic parties for various remarks he made throughout his political career.
Hagel, 66, was first elected to the Senate in 1996, then re-elected in 2002. He retired in 2008, and currently is a professor at the Edmund A. Walsh School of Foreign Service at Georgetown University, the chairman of the Atlantic Council and co-chairman of the Obama's Intelligence Advisory Board.
He co-founded Vanguard Cellular, was president of the investment banking firm McCarthy Group, and chief executive officer of American Information Systems Inc., a computerized voting machine manufacturer, his biography posted on Wikipedia.org indicated.
Hagel was born in North Platte, Neb., graduated from Brown Institute for Radio and Television in 1966, and later earned a bachelor's degree in history from the University of Nebraska at Omaha in 1971.
Some critics challenged Hagel's support of Israel, with Sen. Lindsey Graham, R-S.C., saying he would be "the most antagonistic" secretary of Defense toward Israel ever and called Obama's decision an "in-your-face" nomination.
On the left, Hagel drew criticism for opposing President Bill Clinton's nomination of James Hormel as the U.S. ambassador to Luxembourg by calling Hormel "openly, aggressively gay," for which he later apologized.
However, Hagel also has some high-powered supporters, including former Secretary of State Colin Powell and five former U.S. ambassadors to Israel.
Hagel and his wife, Lilibet, live with their daughter, Allyn, and son, Ziller, in McLean, Va.