"This is how you create a lesbian," they charmingly wrote. "Better counteract now."
However, the proud father hit back with a perfect response — highlighting how ignorant the troll's homophobia was.
"So you think you there is something wrong if my daughter was to grow up to be lesbian?" he wrote in response.
"As her father all I want for my daughter is for her to grow up, be happy and be healthy, if she grows up to be gay and that makes her happy so be it.
"You really need to take a look in the mirror and think do you want to be that person who tries to somehow in your twisted way insult a 6-year-old who is clearly happy dressed as her favourite superhero?"
Others came out in support of the parent.
"As a teen I had a lot of male superhero role models," wrote one Reddit user. "I always wanted to be them, but I never dared dress up as them because I'm female and thought people would judge me. Now I do it anyway, and it's a lot of fun. I wish my parents had been like that."
The father responded: "To be honest I don't even see it as a girl dressing up as a boy, I just see my daughter really happy, having fun dressed up as Thor, I'm glad you feel comfortable dressing as your heroes now."