The country has two years to amend or enact laws
A landmark ruling means Taiwan is close to becoming the first country in Asia to legalise same-sex marriage.
On Wednesday, judges in Taiwan's Constitutional Court ruled that the country's current marriage law is unconstitutional, as it discriminates against same-sex couples.
The judges have given lawmakers two years to amend or enact relevant laws.
Lisa Tassi of Amnesty International says: "The judges have today said yes to marriage equality. This is a huge step forward for LGBTI rights in Taiwan and will resonate across Asia.
"Lawmakers must act swiftly to ensure Taiwan becomes the first in Asia to make genuine marriage equality a reality."
A draft bill on same-sex marriage is being considered by the Taiwanese legislature.
Amnesty International is urging lawmakers to legalise same-sex marriage in Taiwan, on the same basis and with the same rights as marriage between couples of different sex.
"As today’s ruling makes clear, whoever you love, everyone is entitled to the same human rights and equal protection under the law," Tassi added.
Gay rights campaigner Chi Chia-wei was one of the petitioners who brought the case to the constitutional court.
"I'm leaping with joy like a bird," he told AFP.
"I hope parliament will prioritise the bill instead of dragging it on for another two years".
Legalisation of same-sex marriage in Taiwan has been on the cards for some time.
Since the removal of martial law there in 1987 the island has pushed democratic reforms. This led to its first national election in 1996.