Taiwan tends to use the Tiananmen Square anniversary to criticise China and urge it to face up to what it did, to Beijing's repeated annoyance. China claims Taiwan as its own territory, to be taken by force if necessary.
Friday marks 32 years since Chinese troops opened fire to end the student-led unrest in and around the square. Chinese authorities ban any public commemoration of the event on the mainland.
Writing on her Facebook page, Tsai said Taiwan's people would not forget what had happened.
"I believe for all Taiwanese who are proud of their freedom and democracy, they will never forget about this day and will firmly stick with their faith, unshaken by challenges," she said.
"We will also not forget about the young people who sacrificed themselves on Tiananmen Square on this day 32 years ago, and that year after year, friends in Hong Kong who always mourn June 4 with candlelight."
Tsai pointed to the appropriateness of Friday being the same day 1.2 million Covid-19 vaccines will be arriving from Japan as a government donation.
"We are grateful for the timely assistance from partners who also uphold the values of freedom and democracy, so that democratic Taiwan has more confidence in democracy."
Taiwan's Mainland Affairs Council on Thursday urged China to return power to the people and embark on real political reform rather than avoid facing up to the crackdown.
In a statement sent to Reuters, China's Taiwan Affairs Office said the island's government was "smearing and attacking" China when it should be focused on fighting a spike in domestic Covid-19 cases.
"In the face of increasing coronavirus infections and death, this veil they are using to attack others is a bit too much."
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