The 27-year-old performed a jig to celebrate with the runner-up, Kenya's Julius Kipyego Keter, who finished just three seconds behind, because "I'm so happy".
Belachew slashed almost a minute off the course record to cross the line in two hours, 10 minutes and 31 seconds.
"I was not expecting to break the course record but after 33 kilometres I thought it is possible - and I did it," he said.
Belachew, who paid his own expenses to join the race, now plans to buy a house in the Ethiopian capital Addis Ababa with his US$65,000 winner's cheque.
Belachew added of his Kenyan rival: "I didn't know him before but he's my friend now. We celebrated together."
Ethiopia also claimed first place in the women's race, with Gulume Tollesa Chala crossing the line in 2:33:39.
Last year's men's champion, Mike Kiprotich Mutai of Kenya, came sixth and told AFP the recent loss of his mother took the shine off his performance.
"I didn't train for the last two weeks," he said. "But I didn't want to drop out. I was running for my family."
The 42 km course was run under blue skies Sunday in contrast to the heavy rain which drenched last year's event.
