"It is with great sadness that we announce that our grandmother, Madelyn Dunham, has died peacefully after a battle with cancer," Obama said in a joint statement with his sister, Maya Soetoro-Ng. "She was the cornerstone of our family, and a woman of extraordinary accomplishment, strength and humility."
Dunham, 86, helped raise Obama from the age of 10 while his mother was working in Indonesia, and Obama took an emotional 22-hour trip to Hawaii to visit her on October 23 and 24.
Obama said afterward his grandmother had been flooded with cards, flowers and well-wishes from around the country, and he regularly thanked crowds at his campaign rallies for their prayers.
"Our family wants to thank all of those who sent flowers, cards, well-wishes and prayers during this difficult time," the statement said.
"It brought our grandmother and us great comfort. Our grandmother was a private woman, and we will respect her wish for a small private ceremony to be held at a later date," the statement said.
Dunham had followed Obama’s presidential bid with great interest, and her death comes one day before U.S. voters will render their verdict in the race between Obama and Republican John McCain.
Obama affectionately called her "Toot" — short for "tutu," the Hawaiian word for grandmother — and frequently spoke of her on the campaign trail.
Dunham had recently broken her hip but the campaign had refused to comment on reports she was suffering from cancer.