It is the work of street artist Joe Caslin, who rose to public attention during last year's same-sex marriage referendum in the Republic of Ireland.
The Belfast artwork is similar in style to another huge mural of his in Dublin, which depicts two young men embracing.
The artist said he felt the time was right to take the campaign to Belfast.
"Northern Ireland is the only territory now on the islands of western Europe that doesn't have same sex marriage - and it's same-sex marriage not civil partnership," he told BBC Radio Ulster.
"Through the work that I did in Dublin last year with the referendum and the large piece we put up on George's Street, I felt it was an ideal time, coming up to the Pride Festival that takes place this weekend, to reignite that conversation."
The street artist, from County Roscommon, also works as an art teacher at a secondary school in County Offaly.
He said he received a positive reaction from many members of the public in Belfast who stopped to watch him work on the mural in Hill Street.
The new mural, entitled Love Wins, features an embrace between two women who live in Belfast.
"They travelled to America to get married last year," the artist said.
The pair could not have held the ceremony in their home town.
Same-sex marriage became legal in England and Wales in March 2014 and in Scotland, the first gay weddings took place on 31 December 2014.
In May 2015, the Republic of Ireland became the first country in the world to legalise same-sex marriage through a popular vote, with the law taking effect in November last year.
Earlier this month, a couple from the Isle of Man became the first to change their civil partnership into a marriage on the island.
The Northern Ireland Assembly has held a number of votes on the issue and last year, for the first time, a majority voted in favour by the narrowest of margins.
MLAs supported the motion by 53 votes to 52, but it was blocked after the Democratic Unionist Party (DUP) deployed a petition of concern that required the proposal to achieve a cross-community majority.
Those who oppose same-sex marriage have argued that marriage should be between a man and a woman.
They have claimed legalising same-sex marriages undermine family law and a child's right to father and a mother.
A five-storey mural depicting a married lesbian couple has been painted on a city centre building in Belfast, as part of the same-sex marriage campaign.