More than 1,000 same-sex couples will be able to convert their civil partnerships to marriage.
It comes after new regulations were introduced to Parliament today by Northern Ireland minister Robin Walker.
Campaigners have celebrated the move as the "last stage in achieving full marriage equality".
Same-sex couples were able to marry from February, however those in civil partnerships could not apply to convert their unions to marriages.
The latest law change will come into force on December 7 with the first civil partnership conversions expected to take effect on that date.
Couples in civil partnerships will have a three-year window during which they can convert to married status through a simple administrative process. Fees will be waived during the first year.
Cara McCann, Director of Here NI, a group which advocates for lesbian and bisexual women in Northern Ireland and a member of the Love Equality coalition, described the move as a "relief".
"This means that people in civil partnerships, like my partner Amanda and I, can finally become the married couple we have always wanted to be," she said.
"This change means so much to so many people here, as we can finally see that the law respects our relationships as equal."
John O'Doherty, Director of the Rainbow Project, part of the Love Equality coalition, said he and his partner Martin are also among more than 1,000 same-sex couples that can convert their civil unions.
"The campaign for LGBT+ rights does not end with marriage, but the huge support we received from the general public on this issue shows that Northern Ireland demands equality for all," he said.
Patrick Corrigan, Northern Ireland director of Amnesty International, part of the Love Equality coalition, said it is a "huge day of celebration".
"This law change is the final piece of the jigsaw in bringing marriage equality to Northern Ireland," he said.
"It's a huge day of celebration. We fought to change the law so it would cherish all couples and all families equally and now we have achieved that - first with civil marriage, then religious marriage and now finally, with civil partnership conversion.
"We want to thank all our supporters at Westminster and Stormont, including ministers and officials, who helped us to deliver this change. And to all the couples, their families, friends and the wider community in Northern Ireland who gave this campaign unstoppable momentum - this win is yours."
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