Cory McLean said his daughter Marlee and three step-brothers all stripped to the waist Saturday but only she was told to put on a shirt by a lifeguard, who cited city policy that prohibits girls over the age of four going topless at city pools.
The girl, who didn't understand why she was being singled out, put her top back on so she could continue playing, he said.
"She was so embarrassed and really nervous and scared because it appeared she was in trouble," McLean said.
Marlee broke a "sexist and antiquated" rule, said her dad, who called for it to be changed.
City officials said the policy is meant to balance the safety and comfort of everyone using public facilities such as pools.
"Not everyone has the same comfort level with females being topless," said Kristene Scott, the general manager of parks and recreation.
The city doesn't believe the rule violates any human rights or legal ones, Scott said. Nor was it meant to shame Marlee, she said.
"We certainly apologize if the little girl was embarrassed or felt that way — that is not the intent of our policy," she said.
"We try our best to balance a number of factors including our community standards, legal rights of our employees (and) citizens, cultural norms and the rights and freedoms of all of our patrons."
Scott said city policies are automatically reviewed when such incidents occur and staff are looking at best practices in other municipalities.
When the rule was first proposed in Guelph city council, officials said topless women who refused to cover up at public pools would be asked to leave, but if that didn't work, that would be the end of it.
A 1996 appeal court ruling granted women the right to bare their breasts in public after Gwen Jacob, a 19-year-old Guelph university student, was charged with committing an indecent act when she walked home shirtless on a hot summer day five years earlier.
Her lawyer had argued Jacob was being punished for doing exactly what men do.
Police had acted on a complaint from a mother whose young children had seen Jacob walking without a top.