The laws passed by the US congress in 1979 required Washington to provide Taiwan with means of self-defence even as the United States switched diplomatic recognition to Beijing.
Former US House speaker Paul Ryan led the delegation to the island this week, which included congressmen and senior officials, to mark the anniversary of the enactment of the Taiwan Relations Act alongside Taiwan's president Tsai Ing-wen.
Since the legislation was passed, Washington has remained Taipei's most powerful unofficial ally and its leading arms supplier. It manages its relations through the American Institute in Taiwan (AIT).
"Ours is a friendship grounded in history, shared values, and our common embrace of democracy, free markets, the rule of law, religious freedom and human rights," Ryan said at a ceremony at AIT's new office complex in Taipei.
"And our commitment to Taiwan's security remains rock solid."
He hailed the 40th anniversary of the legislation as a "tremendous milestone" in US-Taiwan relations.
Ryan, 49, was the most powerful Republican in Congress when he served as speaker from October 2015 to January 2019. He currently holds no government position.
"Our security cooperation contributes to regional peace and stability across the Indo-Pacific region," added US de facto ambassador to Taiwan Brent Christensen.
Chinese military aircraft, including Su-30 and J-11 fighter jets, flew over the waters off southern Taiwan earlier Monday in the latest of a recent string of military drills around the island.
In response, President Tsai accused China of "challenging stability in the Taiwan Strait".
Donald Trump's administration has sought to strengthen ties with Taiwan. It announced plans last year to sell it $330 million spare parts for several aircraft including the F-16 fighter and the C-130 cargo plane.
Trump also signed legislation paving the way for mutual visits by top officials and the US government approved a licence required to sell submarine technology to Taiwan.
US warships periodically conduct "freedom of navigation" exercises in the Taiwan Strait, the narrow waterway separating the Chinese mainland and Taiwan, which prompt anger in Beijing.
China has stepped up military and diplomatic pressure on Taiwan amid worsening ties since Tsai came to power in May 2016, as she has refused to acknowledge Beijing's claim that Taiwan is part of "one China."