Tsai arrived at John F. Kennedy International Airport on Wednesday. Her next destinations are Guatemala and Belize.
Tsai issued a statement as she set off on her trip, thanking democratic partners for supporting Taiwan. She said she will share a message that Taiwan firmly believes in, and will defend, the values of freedom.
White House National Security Council spokesperson John Kirby mentioned Tsai during a news conference on Wednesday. He said that she is transiting the US and not visiting, adding her planned stopovers were private and unofficial.
"The People of Republic of China should not use the transit as a pretext to step up any aggressive activity around the Taiwan Strait," he said.
When the then US House Speaker Nancy Pelosi visited Taiwan last year, China responded by conducting large-scale military drills near Taiwan.
Tsai is expected to stop in Los Angeles next week for a possible meeting with United States House Speaker Kevin McCarthy.
Observers say Tsai wants to show off her administration's close ties with the US. She also hopes a successful overseas tour will gather support for her ruling Democratic Progressive Party in Taiwan's presidential election slated for next January.
Supporters and protesters gather outside Tsai's hotel
About 200 people gathered in front of the Manhattan hotel where she is staying to show their support.
One of them, a Taiwan native, told NHK that she is excited about Tsai's visit and hopes it will lead to peace.
Another person said, "I want to support Taiwan. Taiwan is a democracy that enjoys freedom."
Across the street, about 500 people rallied against the president to express opposition to Taiwan's independence.
One of the protesters told NHK that he did not understand the point of her visit, saying "Unifying the motherland is the best way for Chinese people."
China says it will take countermeasures if Tsai meets US House Speaker
Beijing has already responded sharply to a possible meeting between Tsai and the US House Speaker.
China's Taiwan Affairs Office of the State Council spokesperson Zhu Fenland told reporters on Wednesday that Tsai's party is making excuses for its political desires as it seeks independence by using various opportunities.
Zhu warned that if Tsai meets McCarthy, that would be a grave violation of Beijing's "One China Principle," and would undermine China's sovereignty and territorial integrity.
She added China would consider such a meeting as a provocation to destroy peace and stability in the Taiwan Strait, and said China will take countermeasures if it went ahead.
Taiwan losing diplomatic ties
Tsai's overseas tour aims to bolster Taiwan's global presence. The number of sovereign states that recognize Taiwan recently decreased to 13, the fewest ever.
China and Honduras established diplomatic relations on Sunday after the Central American nation announced a severing of diplomatic ties with Taiwan.
Honduras stated that it recognizes the People's Republic of China as the only legitimate government that represents all of China and that Taiwan is an inseparable part of Chinese territory.
The Honduran government announced on Wednesday that its President Xiomara Castro is planning an official visit to China on date that is yet to be determined.
China claims that Taiwan is part of China. It has been pressuring nations that have diplomatic relations with Taiwan to sever those ties, using aid and investment as a lever. It aims to isolate Taiwan in the international community.
Since the inauguration of the Tsai administration in 2016, Taiwan has lost formal diplomatic relationships with nine countries, including Honduras.
Tsai released a video on Sunday in which she claims: "China has persistently used any and all means to suppress Taiwan's international participation, intensify its military intimidations against Taiwan, and disrupt regional peace and stability." She added the people of Taiwan "do not give in to threats."