Defar, the 2004 Olympic champion, overtook Dibaba in the final stretch and held on to win the gold medal in 15 minutes, 4.25 seconds.
World champion Vivian Cheruiyot of Kenya surged into second place in the last 50 meters to claim the silver medal in 15.04.73. Dibaba finished with the bronze in 15:05.15.
Dibaba was aiming to repeat as double Olympic champion in the 5,000 and 10,000 meters. She won the distance double in Beijing in 2008 and retained her 10,000 title on the first night of the track and field competition at the London Games.