Dunham, who lived in Hawaii, died on the eve of an election in which her grandson was leading in the race to become president.
"She was the cornerstone of our family, and a woman of extraordinary accomplishment, strength, and humility," Obama said in a statement released by his campaign. "She was proud of her grandchildren and great-grandchildren and left this world with the knowledge that her impact on all of us was meaningful and enduring."
Obama flew to Hawaii for two days in the final stretch of the campaign to be with the woman he called "Toot," short for "tutu," the Hawaiian word for grandmother. He has credited her with his success in life, saying she "poured everything she had into me."