Obi (帯, おび, , literally "sash"?) is a sash for traditional Japanese dress, keikogi worn for Japanese martial arts, and part of kimono outfits.
The obi for men's kimono is rather narrow, 10 centimetres (3.9 in) wide at most, but a woman's formal obi can be 30 centimetres (12 in) wide and more than 4 metres (13 ft) long. Nowadays, a woman's wide and decorative obi does not keep the kimono closed; this is done by different undersashes and ribbons worn underneath the obi. The obi itself often requires the use of stiffeners and ribbons for definition of shape and decoration.
There are many types of obi, most for women: wide obis made of brocade and narrower, simpler obis for everyday wear. The fanciest and most colourful obis are for young unmarried women. The contemporary women's obi is a very conspicuous accessory, sometimes even more so than the kimono robe itself. A fine formal obi might cost more than the rest of the entire outfit.
Obis are categorised by their design, formality, material, and use. Informal obis are narrower and shorter.