Folau tweeted: "I love and respect all people for who they are and their opinions. but personally, I will not support gay marriage."
He was immediately inundated with dozens of replies criticising him for his stance.
Folau is believed to be the first high-profile sportsperson to have publicly announced their opposition to same-sex marriage.
It comes after numerous sporting organisations - including the Australian Rugby Union, the NRL, Cricket Australia and Football Federation Australia - declared their support for a 'yes' vote in the government's postal survey.
