The marriage equality bill, which was pushed ahead after the country's 'Yes' vote in the postal survey, was passed by MPs in the House of Representatives on Thursday afternoon.
While the law is effective immediately, same-sex couples will be able to marry from January 8, 2018, as marriage licences take one month to process.
Couples who previously went overseas to wed are now legally recognised as married.
The bill was passed after over 100 MPs weighed in, and a discussion today of possible amendments about religious protections, the ABC reports.
The only no votes were recorded by Coalition MPs Russell Broadbent, Keith Pitt, David Littleproud and independent MP Bob Katter.
"What a day! What a day for love, for quality, for respect! Australia has done," Prime Minister Malcolm Turnbull said just before the bill was passed.
"The Parliament has got on with it and we have voted today for a equality, for love, it is time for more marriages, more commitment, more love, more respect, and we respect every Australian who was voted, those who voted yes, and those who voted no, this belongs to us all, this is Australia!"
Opposition Leader Bill Shorten added: "I say the gift that LGBTIQ Australians have given all of us is that we are a nation who includes all of our people, who values all relationships and all families, that we are a better nation altogether.
"So, as it is written that there is a time for everything, and a season for every activity under the heavens, it is now time to heal. A time to build, a time to love, a time to love, and is now at last a time for marriage equality."