The agreement has been hailed as a means to ease the damage that unresolved Indian land claims have inflicted on the resource-based economy of British Columbia, but has also been condemned for eroding the rights of the Canadian province's non-native residents.
"Today we make history as we correct the mistakes of the past and send a signal of hope around the world," said Chief Joseph Gosnell, who attended the ceremony in native dress with a carved headdress symbolising the eagle.
More than 500 Nisga'a members, most adorned in the red and black costumes of four clans, sang songs of celebration and danced outside the new community hall in New Aiyansh where the initialling took place on Tuesday. One man held the photograph of an ancestor who helped take the Nisga'a complaints to government officials in the 1880s.
The only news to marr the celebration came later in the day when word arrived of an aircraft crash, at the nearby Nisga'a village of Kincolith, in which five people were killed.
But, this tragedy apart, it was "a good day", as one young girl was heard to say to a friend at the celebrations.
The agreement would cede to the Nisga'a 745 square miles of land at the Nass River, near the Alaska panhandle, with self-government rights, and compensation of some 490 million Canadian dollars (pounds 200m).
The colourful ceremony, with tribal, federal and provincial officials, is only the start of a long ratification process. The treaty must be approved by the 5,500 or so Nisga'a, and by Canada's Parliament and British Columbia's provincial legislature.
The ratification process has been under attack since negotiators gave the treaty "handshake" approval in mid-July, with critics demanding a province-wide referendum.
Critics also complain that giving the Nisga'a law-making powers in their territory will give them special rights over non-Indians in the area.
"This government has no right to change the way we live together without public consent," Raef Mair, a conservative Vancouver radio talk-show host, complained in a commentary.
Indians make up only 3.8 per cent of British Columbia's 3.7 million people. The tribe's path to this treaty has been as rough as the road to New Aiyansh - a long gravel road that crosses a lava flow that killed 2,000 Nisga'a in the 1770s.
Although Sir Francis Drake first claimed their region for England in 1579, the first recorded European contact with the Nisga'a came in 1793 when the British sea captain, George Vancouver, encountered their tribal canoes while mapping the coast.
The Nisga'a and other British Columbian tribes were soon complaining about the loss of traditional fishing and hunting grounds, but Canadian and provincial officials responded by outlawing their right to pursue land claims. "We had all the land. It was arbitrarily taken from us," Roderick Robinson, a Nisga'a leader and negotiator, said.
Although the rules against land claims were repealed in the 1950s, the negotiations that produced this treaty did not begin until 1976, and British Columbia declined to join federal and tribal negotiators until 1991.
If ratified, the treaty would be the first comprehensive land-claims settlement in British Columbia this century.
Glen Clark, Premier of the province, has called the treaty a "template" for talks under way with nearly 50 other tribes.
"We must continue our work to renew the treaty process to ensure it delivers to all First Nations the opportunities that will flow to the Nisga'a," Mr Clark told the gathering.