More than 20,000 people gathered in Sydney to campaign ahead of a non-binding postal vote on changing Australia's marriage act.
Mr Turnbull made a surprise appearance and speech at the launch of the New South Wales Yes campaign.
Opposition leader Bill Shorten then addressed the crowd at the main rally.
The non-binding vote to gauge support for changing Australia's Marriage Act will be sent out from 12 September, with a result expected in November.
The survey will not have the power to legalise same-sex marriage but it could lead to a vote in parliament if the majority of Australians support the change.
The postal plebiscite has been criticised by opponents as an unnecessary delay in resolving the debate, which they say could be settled immediately in parliament.
The Prime Minister previously said he would personally be voting in favour of same-sex marriage, but has not publicly campaigned prior to the surprise speech on Sunday.
"Throughout my public life I've sought to ensure same-sex couples are not discriminated against and their entitlements, be it in respect of medical benefits, taxation, superannuation or employment, are no different to those afforded to heterosexual couples. Why then shouldn't those same rights now be extended to marriage?"
Mr Turnbull pointed out that 23 other countries had already legalised same-sex marriage.
"In any one of those nations, has the sky fallen in? Has life as we know it come to a halt? Has traditional marriage as we know it been undermined? The answer is no," he said.
Meanwhile, Mr Shorten, who is the leader of the Australian Labor Party, told campaigners he anticipated the yes vote to win in the postal survey.
"We've got one last mountain to climb before we make marriage equality a reality. Let's climb it together, today," he said.
His party have been opposed to the plebiscite vote launched by Mr Turnbull's government, criticising its A$122m (£75m; $97m) cost, its non-binding nature, and arguing it could prompt hate-filled campaigns.
The High Court of Australia last week dismissed two separate objections to the poll by same-sex marriage advocates.
Photo by: EPA Caption: It was previously unclear if Prime Minister Turnbull would campaign for a Yes vote