Switzerland has taken its place between Sweden and Syria as a fully participating member of the United Nations.
After decades on the sidelines, it has pledged to play an active role in the world body.
"The Swiss are here. We've waited a long time," the UN Secretary-General Kofi Annan joked as the Swiss flag was raised alongside the 189 flags of the other member states.
Annan bade the Swiss welcome in all four national languages.
"Switzerland is a vivid example of what the United Nations stands for - a peaceful and multicultural society built on strong democratic foundations," Annan told the large crowd that had gathered to watch the ceremony.
"Today is a celebration not only for the Swiss confederation, but for the entire family of nations," the UN chief added.
No longer observer
Earlier, the UN General Assembly, at the start of its 57th session, had unanimously accepted Swiss membership.
The Swiss delegation was escorted from the observer seat it has occupied for decades to its new place, between Sweden and Syria.
The president, Kaspar Villiger, and the foreign minister, Joseph Deiss, cheerfully waved as the whole assembly hall applauded them to their seats.
The Swiss poignantly bade farewell to the Vatican's representative, alongside whom the Swiss have sat as observers for decades.
When East Timor joins the UN in two weeks' time, the Vatican will be the only recognised state outside the world body.
"This is one of the most important moments of my life," Villiger told swissinfo. "Most of all, I'm proud of the Swiss people for voting to join the United Nations."
The Swiss people opted to join the UN in a nationwide vote on March 3, and the 80-strong Swiss delegation has been emphasising that the country, while being one of the last to join, is the first to decide to do so as a result of direct democracy.
In the previous vote on the issue - in March 1986 - 75 per cent of Swiss voters rejected membership of the UN, in the face of government support for joining.
"This is an important day for our country," Foreign Minister Joseph Deiss told swissinfo, saying that Switzerland could bring added value to the United Nations in areas where it had specific expertise, such as human rights and sustainable development.
If there is satisfaction and jubilation among the Swiss delegation, their celebrations have been tempered by the fact that New York is preparing to commemorate the first anniversary of the attacks on the World Trade Centre.
All along, the Swiss have been in contact with their hosts to ensure that the events they have planned are appropriate.
But in a gesture towards the new UN member, the Empire State Building - which for the past year has been lit up at night in a patriotic red, white and blue - was illuminated in red and white.
Villiger stressed that Switzerland had worked very closely with the United Nations for many years.
Even if it had only been an observer in the UN's political bodies - the General Assembly and Security Council - it had been a full member of most of its specialised agencies, as well been a generous funder.
"Being able to join the decision making in the political United Nations is an important step. But it won't change our foreign policy. It just gives us a new instrument to pursue our foreign policy," the president said.
Until March, the Swiss had always rejected UN membership, feeling that the country was a special case, and that its neutrality would be undermined by the compromises it would have to make in the UN.
Villiger said that Switzerland would remain a "special case", thanks to its direct democracy. Furthermore, he added, joining the UN was on the strict understanding that it did so as a neutral country.
But, the president said, the Swiss people had finally realised that Swiss neutrality could not immunise the country against global events.
"We are not an island on this planet," Villiger told swissinfo. "We have an interest in standing together with the rest of the world."
It is customary for new members to present a gift to the UN. Deiss announced that Switzerland would renovate the room where heads of state wait and prepare themselves before addressing the General Assembly.
swissinfo, Roy Probert in New York
In 1986 the Swiss rejected UN membership by a clear majority.
Switzerland finally voted to join the UN in March this year.
It will be one of 190 members.
Switzerland contributes around SFr500 million annually to UN organisations.