The traditional loincloth worn by sumo wrestlers is called a mawashi.
A mawashi is a long, sturdy belt wrapped around the wrestler’s waist and groin. It is the only garment worn during a professional sumo bout. Wrestlers use the cloth both as protective clothing and as part of the grappling contest, because competitors may seize an opponent’s mawashi to throw or force them out.
Mawashi vary by setting. In tournaments and official bouts, wrestlers typically wear a firm silk mawashi, while training commonly uses a cotton version. Top-division wrestlers may wear colored mawashi, whereas lower-ranked competitors more often wear dark tones. A wrestler’s mawashi must be tied securely before he enters the ring.
The mawashi should not be confused with a modern loincloth worn for ordinary exercise. Its construction, knot and decorative apron-like front strips are all associated with sumo’s formal visual tradition. Losing or loosening it during a bout is extremely unusual and can lead to disqualification.