The image, posted on Facebook but since deleted by the Vote NO Australia group, featured Trainor alongside the caption: “My vote is no, you need to let it go.”
A fan took to Twitter to inform the entertainer of the campaign.
Trainor responded with a series of tweets, saying her image was being used "illegally" while also stating her support for marriage equality.
Celebrity chorus
Trainor's support was the latest in a string of celebrity endorsement for the 'Yes' vote in the upcoming Australia postal survey around marriage equality, which asks the question: "Should the law be changed to allow same-sex couples to marry?"
Kylie Minogue has joined Hollywood stars Chris Hemsworth and Russell Crowe in backing a vote to allow gay marriage in Australia.
The three celebrities have urged their countrymen and women to ensure they are signed up for the postal survey.
Hemsworth posted on Instagram: "Dear open minded, free speaking, laid back, life loving aussies. Marriage is about love and commitment and, in a country based on equal citizenship, it should be available to everyone! Vote now for marriage equality."
Crowe tweeted: "Hey Australia , I love you. Thanks for being you. Please send a clear message with your vote and your voice ... love is love."
Stephen Fry, Tim Minchin and Ellen DeGeneres were other famous faces who urged voters to register and back the call for marriage equality.
DeGeneres posted: "Australia, I love you. It's time for marriage equality. Make sure you're enrolled to vote right now," while Fry called on people to support their LGBTIQ friends, family and community.
'Muddy waters'
Australia's Defence Minister Christopher Pyne used an appearance on the ABC's 7.30 program on Thursday to expound a perspective on same-sex marriage seemingly at odds with the conservatives within his own party.
"It's got nothing to do with freedom of speech, nothing to do with religious freedom," Minister Pyne said.
"What the people who are advocating a 'no' vote are doing with that red herring, of course, is trying to make the question about anything other than the simple question of: do you think people of the same sex should have the same rights as people of opposite sex?"
The result of the vote will be revealed on November 15.
-With AAP
