The 19-year-old singer revealed he started stripping down in the studio when the group were working on their hit 'What Makes You Beautiful' and he has continued the trend ''ever since''.
Speaking to Heat magazine, he said: ''I've been doing it for a while. I recorded 'What Makes You Beautiful' in my boxers.
'' 'What Makes You Beautiful' was the first time I did it because it was really warm in the studio, and I've done it ever since.''
While Niall might be open with his band mates, he has admitted in the past that it can be ''embarrassing'' when their fans are similarly open.
Louis Tomlinson previously revealed they were shocked when excited girls in Sweden flashed their breasts as the group were driving past.
He said: ''The Swedish fans were pressing their breasts up against the car window.''
Niall added: ''It was quite embarrassing.''
He also recently made a more innocent revelation when he named 'Harry Potter' star Tom Felton as the actor he'd most like to play him in a potential biopic.
The pair teamed together for another round of S&M romance in 'Fifty Shades Darker'.
The musician worked with screenwriter Johnny Harris on the new movie.
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