"Make no mistake. Justice will be done," he said in an appearance at the Rose Garden outside the White House, where he was joined by Secretary of State Hillary Rodham Clinton.
Obama, who ordered an increase in security at U.S. facilities overseas, said he "condemns in the strongest possible terms the outrageous and shocking" attack.
He spoke after Republican presidential challenger Mitt Romney condemned the attack, and criticized the administration for its initial response to a separate incident on Tuesday, the breach of the U.S. embassy in Cairo.
The attacks occurred Tuesday night in the eastern city of Benghazi by protesters angry over a film that ridiculed Islam's Prophet Muhammad, according to Libya officials. Ambassador Chris Stevens, 52, was killed when he and a group of embassy employees went to the consulate to try to evacuate staff as the building came under attack by a mob guns and rocket propelled grenades. Three other Americans were also killed.
Stevens is the first U.S. ambassador to be killed in an attack since 1979, when Ambassador Adolph Dubs was killed in Afghanistan.
The State Department identified one of the other Americans killed Tuesday as Sean Smith, a foreign service information management officer. The identities of the others were being withheld pending notification of next of kin.
The White House said Obama was notified Tuesday night that Stevens was unaccounted for and was told Wednesday morning that Stevens had been killed.
Obama was informed about the developments in Libya by his National Security Adviser Tom Donilon as the president began a weekly meeting Secretary of Defense Leon Panetta and Chairman of the Joint Chiefs of Staff Martin Dempsey. The White House said Obama was kept apprised throughout the evening and then again Wednesday morning.
The Pentagon said early Wednesday that it was working with the State Department on Obama's order for increased security around the world.
"We are following this tragic incident closely with the State Department," Lt. Col. Steven Warren, a Defense Department spokesman said. "We are prepared to support the State Department in any way."
Speaking earlier at the State Department, Clinton said those killed had been "committed to helping the Libyan people reach for a better future."
The U.N. Security Council has a long-scheduled meeting Wednesday morning to discuss Libya and diplomats said the United States is seeking a council statement on the attack. U.N. Undersecretary-General Jeffrey Feltman, a former American diplomat and close friend of Stevens', is scheduled to brief the council on Libya.
Stevens was a career diplomat who spoke Arabic and French and had already served two tours in Libya, including running the office in Benghazi during the revolt against Libyan leader Moammar Gadhafi. He was confirmed as ambassador to Libya by the Senate earlier this year.
His State Department biography, posted on the website of the U.S. Embassy to Libya, says he "considers himself fortunate to participate in this incredible period of change and hope for Libya."
Clinton said Stevens had a "passion for service, for diplomacy and for the Libyan people."
"This assignment was only the latest in his more than two decades of dedication to advancing closer ties with the people of the Middle East and North Africa which began as a Peace Corps Volunteer in Morocco," Clinton said.
He "risked his own life to lend the Libyan people a helping hand to build the foundation for a new, free nation. He spent every day since helping to finish the work that he started," she said.
Stevens joined the Foreign Service in 1991 and spent his early State Department career at posts in Saudi Arabia, Egypt, Syria and Israel. After working for the Senate Foreign Relations Committee staff for Sen. Richard Lugar, R-Ind., Stevens was posted to Libya as deputy chief of mission.
In that post, Stevens wrote several confidential cables back to Washington, describing Gadhafi's bizarre behavior. During the 2011 revolt against Gadhafi, he was one of the last American diplomats to stay in Tripoli and after the embassy was closed, he was appointed to head the U.S. liaison office to the Transitional National Council.
Associated Press writer Edith M. Lederer contributed to this report.