"It was unknowable then, but so much of the progress that would define the 20th century, on both sides of the Atlantic, came down to the battle for a slice of beach only six miles long and two miles wide," he said.
"Had the Allies failed here, Hitler's occupation of this continent might have continued indefinitely."
In what remains the biggest amphibious assault in history, some 156,000 Allied personnel landed in France on June 6, 1944.
Cost of war
An estimated 10,000 Allied troops were left dead, wounded or missing, while Germany lost between 4,000 and 9,000, and thousands of French civilians were killed.
"We will never forget the pain or the extent of the suffering and may we never renounce the dream of peace and justice for humanity," Nicolas Sarkozy, the president of France, said alongside Obama, Canadian Prime Minister Stephen Harper and British Prime Minister Gordon Brown.
Britons accounted for the lion's share of the 1,500 ex-servicemen spread across ceremonies in Normandy and Brown paid tribute to "the brave fighting men of the largest amphibious operation in the annals of warfare."
"This day marks the triumph of right over wrong... the victory of human decency over hatred and the Holocaust," Brown added.
Among guests at the ceremony was Tom Hanks, star of "Saving Private Ryan" which begins with graphic scenes depicting the Normandy landings.
Former Republican presidential candiate, senator and wounded World War II hero Bob Dole was there as was Charlie Payne, Obama's great uncle, who took part in the liberation of the Buchenwald Nazi concentration camp in Germany which Obama visited on Friday.