Same-sex marriage is now legally recognised in Northern Ireland.
From Monday, same-sex couples will be able to register to marry, meaning the first ceremonies will take place in February.
For couples who are already married, their marriage will now be legally recognised in Northern Ireland.
However, those who are already in a civil partnership will not be able to convert it to a marriage at this stage.
The Northern Ireland Office is set to begin a consultation later this year about converting civil partnerships and the role of churches in same-sex marriages.
Heterosexual couples will also be able to enter into civil partnerships from today.
When the Stormont assembly collapsed, marriage equality campaigners turned their focus to Westminster.
In July 2019, MPs backed amendments which required the government to change abortion laws and extend same-sex marriage to Northern Ireland if devolution was not restored by 21 October 2019.
An amendment was made to the Northern Ireland (Executive Formation etc) Act 2019 by the Labour MP Conor McGinn saying that the government had to legislate for same-sex marriage in Northern Ireland.
Speaking to BBC News NI, Mr McGinn said "everyone who values equality, love and respect can celebrate today".
"It's a good day for Northern Ireland, an important day for citizens' rights across these islands and an exciting day for same-sex couples who can now register to marry," he said.
Patrick Corrigan from Amnesty International said it was a "historic day for equality and human rights in Northern Ireland".
"For too long, LGBT+ people in Northern Ireland have been treated as second-class citizens. So, today is an incredible moment for same-sex couples who can finally marry and have their relationships recognised as equal," he said.