The U.S. special envoy for Afghanistan and Pakistan, Richard Holbrooke, died Monday
He fell ill on Friday while meeting US Secretary of State Hillary Clinton.
Mr Obama called the 69-year-old, who was known for bringing warring leaders to the negotiating table, a "true giant of American foreign policy".
He was rushed to the US capital's George Washington University Hospital, where he underwent surgery to repair a tear in his aorta - the largest artery in the human body, which carries oxygenated blood from the heart.
Following the news of his death, Mr Obama said: "Michelle and I are deeply saddened by the passing of Richard Holbrooke, a true giant of American foreign policy who has made America stronger, safer and more respected.
"He was a truly unique figure who will be remembered for his tireless diplomacy, love of country, and pursuit of peace."
Ms Clinton also paid tribute to Mr Holbrooke's service, saying the US had lost "one of its fiercest champions and most dedicated public servants".
Beginning with a foreign service posting in Vietnam in 1962, the veteran diplomat served under every Democratic president from John F Kennedy to Mr Obama.
Nicknamed "the Bulldozer", the veteran diplomat once said he had no qualms about "negotiating with people who do immoral things", if it served efforts for peace.
Mr Holbrooke was named by Mr Obama as his special representative to Afghanistan and Pakistan shortly after the president took office in January 2009.
Хотели бы вы заключить прижизненный договор?
А что это?
Да, если бы мне предложили
Никогда не задумывался об этом
Ни в коем случае
- А что это?