A tatami (畳^?) is a type of mat used as a flooring material in traditional Japanese-style rooms. Traditionally made of rice straw to form the core (though nowadays sometimes the core is composed of compressed wood chip boards or polystyrene foam), with a covering of woven soft rush (igusa) straw, tatami are made in standard sizes, with the length exactly twice the width, an aspect ratio of 2:1. Usually, on the long sides, they have edging (heri) of brocade or plain cloth, although some tatami have no edging.

The term tatami is derived from the verb tatamu, meaning to fold or pile. This indicates that the early tatami were thin and could be folded up when not used or piled in layers.^[1] Tatami were originally a luxury item for the nobility. During the Heian period, when the shinden-zukuri architectural style of aristocratic residences was consummated, the flooring of shinden-zukuri palatial rooms were mainly wooden, and tatami were only used as seating for the highest aristocrats.^[2] In the Kamakura period, there arose the shoin-zukuri architectural style of residence for the samurai and priests who had gained power. This architectural style reached its peak of development in the Muromachi period, when tatami gradually came to be spread over whole rooms, beginning with small rooms. Rooms completely spread witzashiki (lit., room spread out for sitting), and rules concerning seating and etiquette determined the arrangement of the tatami in the rooms.^[2]goza, while commoners used straw mats or loose straw for bedding.^[3] It is said that prior to the mid-16th century, the ruling nobility and samurai slept on tatami or woven mats called

The lower classes had mat-covered earth floors.^[4]

Tatami were gradually popularized and finally reached the homes of commoners toward the end of the 17th century.^[5]

Houses built in Japan today often have very few tatami-floored rooms, if any. Having just one is not uncommon. The rooms having tatami flooring and other such traditional architectural features are referred to as nihonma or washitsu, "Japanese-style rooms".