It has secured the 3,557ft summit and 4,000 acres for the nation as a whole, and for Wales in particular, in a deal with farmer Richard Williams, whose family has farmed the land for 14 generations.
Last month Mr Williams put on the market his two estates on the southern flank of the mountain, Hafod-y-Llan and Gelli Iago, after deciding he could not continue to farm them while running another estate in Anglesey.
He said then that the decision had caused him "much heartache" and that he would prefer to sell to a fellow Welshman.
The sale attracted widespread interest but in the end there were only three serious bidders besides the National Trust: the Snowdon Trust (an ad hoc group of Welsh businessmen), the Countryside Restoration Trust (a charity run by countryside journalist and broadcaster Robin Page), and an unnamed foreigner.
The National Trust, with its annual income of pounds 175m as the largest conservation organisation in Europe (membership now over 2.5m), was clearly in the strongest bargaining position.
The deal was closed by Mr Williams' agents, Carter Jonas, on Tuesday night.
After the National Trust's offer (thought to be in the region of pounds 3.5m) was accepted, immediately it launched a campaign to secure the funds.
Although its income is vast, its expenditure on its hundreds of historic properties was last year even greater at pounds 180m.
The actor Sir Anthony Hopkins, president of the trust's Snowdonia Appeal, said there were 100 days to raise the money.
"There is no time to lose and every contribution will count," he said. "Snowdon is an important part of our Welsh heritage. Together we must ensure its future for everyone to enjoy."
Lord Dafydd Elis Thomas, the appeal's vice-president, said: "We are delighted the negotiations have been successful and we now have the opportunity to acquire Snowdon for the nation. We must now raise the funds so that we can complete the deal.
"To make this happen we urgently need financial support," he said.
"I have every confidence that individuals and organisations both within and outside Wales will contribute to this unique opportunity."
The mountaineer Chris Brasher promised a donation of pounds 200,000 from the Chris Brasher Trust and challenged all mountaineering and climbing clubs affiliated to the British Mountaineering Council, to contribute, saying: "Rally your members in defence of Snowdon. For every pound you contribute we will add another pound."
The Trust already owns and cares for 10 of the highest peaks in Snowdonia.
Carter Jonas spokesman Tony Pardoe said Snowdon had taken a month to sell to the highest bidder after a period of intense interest from both public bodies and private individuals.
"We are delighted that, at the end of the day, the land will be secured for the Welsh nation via the auspices of the National Trust," he said.