Haile, unbeaten over the distance for seven years, ran 27:18.20 seconds for the gold after an epic battle down the final straight with his old rival, Paul Tergat of Kenya.
The winning margin - nine hundredths of a second - was the closest finish in the event in Olympic history.
Haile said: "The reason why I could not run so fast was because I have an injury.
"For a time I didn't know whether I would come here. At the last moment I decided to try and now I'm very happy."
Tergat, the five-times world cross country champion and the 1996 Olympic silver medallist behind the Ethiopian in Atlanta, seemed set to outdo Haile at last when kicking for home on the crown of the bend.
Grimace
But Haile, known for his beaming smile rather than the grimace he showed in the final stages, clung on to his heels before stealing past.
"At the last moment, Tergat started to sprint," Haile said. "I didn't expect it, it was the toughest race ever."
Tergat clocked 27:18.29 while Assefa Mezgebu (27:19.75) made it an all-African podium with third place.
The 27-year-old Haile, who weighs just 53 kg, has dominated his event since 1993, when he won the first of four consecutive world titles.
However, his season had been interrupted by an ankle injury and many wondered if he would be at the peak of his powers.
Relaxed
Haile was clearly unconcerned as he stood at the start, looking totally relaxed and smiling.
And despite all his efforts, Tergat was not able to wipe that smile away. The Kenyan said: "It was very, very hard. I'm satisfied with the silver because I gave my very best."
Asked if he would be having nightmares about the seemingly unstoppable Ethiopian, he said: "No. I'm an athlete and he's also a human being. "He's a really good guy. No nightmares."
Tergat added: "I believe this was my last race on the track. Now I will run marathons."
The bad news for the Kenyan is that Haile plans to do the same.
British hopeful Keska Karl ran another personal best of 27:44.09 as he came home an unexpected 8th in his first major final.
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