Tatami mats (畳) (originally meant “folded and piled”) are a traditional Japanese flooring. Made of woven straw, and traditionally packed with straw (though nowadays sometimes with styrofoam), tatami are made in individual mats of uniform size and shape, bordered by brocade or plain black cloth.

Tatami were originally a luxury item for the wealthy at a time when most people had floors made of dirt.

There are various rules concerning the number and layout of tatami mats; an inauspicious layout is said to bring bad fortune. The mats must not be laid in a grid pattern, and in any layout there is never a point where the corners of three or four mats intersect.

In Japan, the size of a room is typically measured by the number of tatami mats (-畳, -じょう). Shops were traditionally designed to be 5 1/2 mats (= 8.91 m2), and tea rooms and tea houses are frequently 4 1/2 mats (= 7.29 m2). The traditional dimensions of the mats was fixed at 35.5 in by 71 in by 2 in (90 cm by 180 cm (= 1.62 square meters) by 5 cm). Half mats, 35.5 in by 35.5 in (90 cm by 90 cm) are also made. Because the size is fixed, rooms in traditional Japanese contruction measure in multiples of 35.5 inches. It should be noted that mats from Kyoto and other parts of western Japan are slightly larger than those from Tokyo and eastern Japan at 33.5 in by 70.5 in (85 cm by 180 cm = 1.53 square meters).

Tatami mats are associated with Japanese religious rites and the tea ceremony. Most modern Japanese homes still have at least one tatami room.