The country uses the calendar of the Coptic Orthodox Church - which means the year 2000 began at midnight on Wednesday (2100GMT on Tuesday).
Fireworks lit up the sky above the city, as revellers gathered in the streets for a cultural festival.
A more exclusive event was attended by a number of African leaders, including Kenya's Mwai Kibaki.
In the 6th Century, the rest of Christendom revised its estimate of the date of the birth of Christ - but Ethiopia stuck with the old date, which means it was stuck more than seven years behind the rest of the world.
'Time for renaissance'
Ethiopians gathered together in churches all over the country to mark the occasion.
Speaking at the main event to welcome in the Millennium,
Prime Minister Meles Zenawi spoke of the glories of Ethiopia's first
"A thousand years from now, when Ethiopians gather to welcome the fourth Millennium, they shall say the eve of the third Millennium was the beginning of the end of the dark ages in Ethiopia," he said.
Because it was never colonised, Ethiopia has always had a special significance for the rest of Africa - which was summed up by the chairman of the African Union, Alpha Oumar Konare.
"We thank Ethiopia for keeping up the flame of African unity," he said.
He was speaking at the new exhibition hall where foreign dignitaries gathered.
But correspondents say Ethiopians have mixed feelings about the Millennium celebrations.
Some residents of Addis Ababa - an opposition stronghold - criticised the government's campaign to clear the streets of beggars and the rising cost of food ahead of the celebrations.
Tickets for a concert featuring US hip hop group the Black Eyed Peas cost $170, two months' salary for an average Ethiopian.