The Haskell Free Library and Opera House is between the towns of Stanstead, Quebec, and Derby Line, Vermont.
It has become a symbol of cross-border unity and co-operation as well as an opportunity for separated families to unite.
The library's entrance is on the Vermont side.
Previously, Canadian visitors were able to enter using the footpath and entrance on the American side but were encouraged to bring documentation, according to the library's website.
Under the new rules, Canadians would need to go through a formal border crossing before entering the library.
Until October 1, library members would be able to use the footpath with proof of membership, the US Customs and Border Protection said, after which Canadians will need to enter via a formal border crossing.
"This closure not only compromises Canadian visitors' access to a historic symbol of co-operation and harmony between the two countries but also weakens the spirit of cross-border collaboration that defines this iconic location," the town of Stanstead said in a statement.
Relations between the United States and Canada, long-time allies, have deteriorated since US President Donald Trump threatened to annex Canada as the 51st state and imposed tariffs.
The CBP spokesperson said the area had witnessed a "continued rise in illicit cross-border activity", without specifying the nature of the illicit activity.
But according to CBP data, US authorities appear to be detaining fewer people than they were a year ago, with 21 apprehensions of people near the library so far in the 2025 fiscal year, which began in October, versus 147 apprehensions in the entire 2024 fiscal year.
The library is a relic of a time when Americans and Canadians could cross the border with simply a nod and a wave at border agents, residents say.
It was the gift of a local family in the early 1900s to serve the nearby Canadian and American communities.
Stanstead Mayor Jody Stone told Reuters he was disappointed more than anything.
"Disappointed that we have to change something that's been a certain way for over 100 years. Disappointed in the sense that there's no reason, other than control, for having to do this," he said.
A small group of American and Canadian protesters gathered outside on Friday.
US senator Peter Welch, a Democrat from Vermont, called reports of the closure troubling.
"Vermont loves Canada. This shared cultural institution celebrates a partnership between our two nations," Welch said on X.
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