In a show of how important the American presence is for Taiwan, the island’s president, Ms. Tsai, was joined at the dedication ceremony by her predecessor, Ma Ying-jeou. Many of Taiwan’s top politicians sat in the audience of about 200.

“As we dedicate this building, we also rededicate ourselves to our common sense of purpose,” Ms. Tsai said. “As free and open democracies, we have an obligation to work with one another to defend our values and protect our joint interests.”

When asked by a journalist if he was disappointed that no higher-level representatives were sent from Washington, Mr. Moriarty, the institute’s chairman, pointed to the summit meeting in Singapore.

“I believe there is a lot going on in the region today, and we need to respect that,” he said.

Speaking to a crowd of reporters before the ceremony, the institute’s director, Kin Moy, a career diplomat in the State Department, said the new building’s unveiling showed Washington’s “unshakable” commitment to Taiwan.

After delays since its groundbreaking in 2009, the institute’s fortified compound on the northern edge of Taipei, Taiwan’s capital, will begin operations in September, when some 450 staff members, including several American diplomats, will relocate from their current offices scattered around the city.

The institute performs most of the same functions as an embassy, including issuing visas. It is the largest diplomatic mission — official or unofficial — in Taiwan, making it a symbol of both Taiwan’s awkward diplomatic limbo and also its quiet but strong ties with the United States.

American support for Taiwan goes back to the end of China’s civil war in 1949, when Chiang Kai-shek, the World War II ally of the United States, fled to the island with his Republic of China government after being routed by Mao Zedong’s Communists.