Taking to Instagram, the doting dad shared a photo of his wife Jessica Biel and their son Silas on holiday in Paris.
Jessica is kissing their son lovingly, with the little boy's face covered by his long hair.
Justin captioned: "If that pic doesn't say ‘City Of Love’ then I'm out..."
One fan wrote: "I don’t like when little boys have long hair, and they are too small to decide if they love it," with another adding, "It's like they wanted a girl but when they got a boy they make it look like girl."
However, fans of Justin's rallied behind the singer to defend the parent's decision.
One asserted, "What concern is it of yours? People like you are the reason celebrities don't like putting pictures of their kids up."
"He can make the decision to cut his hair when he is older. There is literally no difference. Hair has nothing to do with gender," another added.