A Catholic primary school principal in Perth has apologised to parents after a student dressed as Adolf Hitler won first place in class on a school dress-up day.

A letter went out to parents on Wednesday after some complained the costume should not have been allowed at the inner-city school.

Children had been asked to dress up as famous people and a year 4 student had turned up as the Nazi leader, complete with swastika.

The school principal said he did not see anything sinister in the student's choice of costume.

"To me it's a mountain out of a mole hill," The West Australian newspaper reported him saying.