On Friday - only a week off the two-year deadline - lawmakers in Taiwan's Legislative Yuan passed a bill making same-sex marriage a reality.
The vote allows same-sex couples full legal marriage rights, including in areas such as taxes, insurance and child custody.
In recent months, conservatives had mobilised to rid the law of any reference to marriage, instead putting forward rival Bills that offered something closer to limited same-sex unions.
The Executive Yuan's version was listed as the first bill to be voted on and legislative rules state that there is no need to vote on other comparable clauses if the previous one is passed.
Tens of thousands of gay rights supporters, who turned out in spite of heavy rain, cheered outside the parliament building in central Taipei after lawmakers voted in favour of the bill.
Despite the spread of same-sex marriage in a few regions since 2001, gay and lesbian couples are permitted to marry in only about two dozen of the world's almost 200 countries.
'On May 17th, 2019 in #Taiwan, #LoveWon.
"I'm pleased that Taiwan has made progress and is sending a message to the world", she said.
Ms Tsai's ruling Democratic Progressive Party (DPP) holds the majority in Parliament, occupying 68 out of 113 seats.
Same-sex marriage supporters celebrate after Taiwan legalizes same-sex marriage, Taiwan May 17, 2019.
Activists said they would continue to push for more rights, such as recognition of transnational same-sex marriages, where one partner is from a country that does not recognise gay marriage.
The law was also celebrated by many LGBT people in the region.
Under that act, even with the legalization of same-sex marriage in Taiwan, a Taiwanese national would not be able to enter into a legally recognized union with a partner from a country where gay marriage is not legal.
"Today, we can show the world that #LoveWins", added Tsai, who campaigned on a promise of marriage equality in the 2016 presidential election.
"For the gay communities what matters the most is whether we can legally get married on May 24 and be listed as the spouse in ID cards, to be treated and respected as the "spouse" in the whole legal system. and whether same-sex families can obtain legal parental rights for their children".
Earlier on Friday, Ms Tsai said in a tweet that the island had taken "a big step towards true equality" with the vote.
The new law will take effect May 24, allowing two persons of the same gender, aged 18 or older, to register a marriage, with at least two witnesses signing the registration document.
Opponents warn that "forcefully" passing a gay marriage law will intensify tensions.
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