In a short statement on Thursday, the Electoral Affairs Commission's Returning Officer, Justice Carlye Chu Fun-ling, said Tsang had been declared victor after emerging as the only candidate from a preliminary nomination round.
Chu said Tsang, who had long been groomed to succeed former leader Tung Chee-hwa, had secured the backing of 674 members of the 800-strong election committee of mostly Beijing loyalists.
He received so much support that he shut out all challengers, echoing the way Tung was elected for his second term in 2002.
Two other would-be candidates pulled out of the race after declaring they had failed to win the support of the 100 delegates each needed to stand.
"There being only one valid candidate at the close of nominations I hereby declare Donald Tsang is returned and elected in the 2005 chief executive election," Chu said.
No formal elections
There will now be no need for a formal election originally set for July among the 800-member committee. Tsang, 61, thanked its members for their support as well as the people of Hong Kong.
"The election has been an exhilarating experience for me. I shall treasure it dearly," he said.
"The dialogue that I have started with various sectors of the community will not be a one-off exercise. It is my pledge that I will continue to keep this very meaningful interchange going in the months and years ahead.
"I hope now that the election is over, we'll all be able to come together joining hands and working for the best interests of Hong Kong"
leader of Hong Kong
"I hope, now that the election is over, we will all be able to come together, joining hands and working for the best interests of Hong Kong," he added.
Tsang, a top civil servant during British colonial rule who was knighted by London, plans to shake up his cabinet but declined to say whether any democrats will be included.
Although Tsang enjoys much higher popularity ratings than his predecessor, analysts say he will inherit many of the huge challenges which Tung faced and left unresolved.
Foremost among them will be the way he handles the territory's popular pro-democracy movement, which has a powerful voice within the legislature.
But he will have to balance that against the demands of the pro-Beijing lobby, which dominates the halls of power in the territory returned to China in 1997.
"Beijing has chosen Tsang to improve relations with the democrats. So he can't act like Tung, who used to always speak in favour of the Chinese government," said Antony Cheung, political science professor at City University.
While both Beijing-backed newspapers Wen Wei Po and Ta Kung Pao hailed Tsang's victory and noted the failure of his opponents, other local newspapers said his brief Western-style campaign highlighted some of the difficulties he will face.
"His overwhelming victory will always be tainted by a perception that many voters backed him to please Beijing," the South China Morning Post said in an editorial.
"The result of all this is that Hong Kong was deprived of a real contest. Mr Tsang will not benefit from the extra legitimacy an election by secret ballot would have given him," it says.
Outgoing Tung Chee-hwa (R)
moves over for Donald Tsang (L)
In the past two weeks, the devout Catholic famed for his trademark bow-ties did everything but kiss babies on the campaign trail - releasing a manifesto, holding public meet-and-greet sessions and courting businessmen and politicians.
But he refused to take part in forums with other would-be candidates. He held meetings with election committee members behind closed doors and failed to investigate social issues, said Ming Pao Daily News.
"He talked more and did little. This was truly disappointing for people," it said.