He told The Sun: "Louis has the smelliest feet ever."
The teenager - who will share a tourbus with Zayn Malik, Harry Styles and Liam Payne when they begin their US tour on Tuesday - says the stench is because of his choice of footwear.
He added: "He wears plimsolls with no socks so his feet get very sweaty and the sweat is captured."
Niall declared that is not only him who thinks Louis doesn't take correct care when it comes to the hygiene of his feet, but people who work with them would agree.
If the 'What Makes You Beautiful' boys are in a closed space and he takes his shoes off, everyone notices because of the bad smell that emerges.
He continued: "If we are on a bus or something and he takes them off we all pretty much start gagging."