With the ruling, Ecuador joins a handful of Latin American nations - Argentina, Brazil, Costa Rica, Colombia and Uruguay - that have legalised same-sex marriage.
Just one day before the move from Ecuador, Botswanna's top court voted to decriminalise homosexuality, and a week ago Bhutan voted to repeal similar laws. LGBT organisations are celebrating the 'good news week'.
International LGBT rights group All Out programs director Matthias Wasik said the "victories we’ve witnessed in the last couple of weeks will improve the lives of millions of LGBT+ people around the world".
“We’re witnessing an important moment in history as these victories will send out positive shockwaves across the world and inspire more activists to continue their fight for LGBT+ rights."
Ecuador has recognised de facto civil unions for same-sex couples since 2015 - but marriage is defined as a union between a man and a woman under the constitution.
In Wednesday's ruling, the justices instructed congress to pass legislation ensuring equal treatment for all under the country's marriage law.
The Patka Foundation provides legal advice for same-sex couples seeking to marry in Ecuador.
A lawyer for the foundation, Christian Paula said the decision means the country "is more egalitarian".
"It recognises that human rights must be for all people without discrimination," he said.
But some organisations, while celebrating the win in Ecuador, say there's still a long way to go in the global fight for equality.
Human Rights Watch senior researcher on LGBT rights Neela Ghosha warned that there's been support globally for leaders with anti-gay views.
“Whenever there’s progress, there’s always steps backwards,” she said.
“Conservatives around social issues, particularly religious conservatives, take progress as a threat to their belief system."
In Ecuador, where the church is very influential, the Life and Family movement, a right-wing Christian group, led much of the opposition to same-sex marriage.
A lawyer for the group, Carlos Arsenio Larco, told local TV channel El Comercio that a majority of Ecuadorian people wanted to reserve marriage for heterosexual couples.
But the LGBTQI community in Ecuador celebrated the news.
LGBTI activist Freddy Veloz Baez said it was a historic day, not just for the couples who had led the court case, but for Ecuador as a whole.
"The Constitutional Court has stood on the right side of history and recognized that all Ecuadorians deserve the same rights, no matter our sexual orientation or gender identity," he said.
"There is still a long road ahead in order to end homophobia in our society, but today we celebrate. For all the couples that will be able to marry and for the future generations, this will be a day to be remembered."
Ecuadorian Federation of LGBTI Organisations president Diane Rodriguez said it's been a long fight to achieve equality.
“After a fight of almost 20 years, gay marriage has been achieved," she said.
"It gives us a guiding light for many other proposals on human rights.”