Seiza (正座, literally "correct sitting") is a Japanese word which describes the traditional formal way of sitting in that country.

To sit seiza-style, one first kneels on the floor, and then rests the buttocks on the heels, with the tops of the feet flat on the floor. The hands are sometimes folded modestly in the lap and sometimes placed palm down on the upper thighs with the fingers close together. The back is kept straight, though not unnaturally stiff. Traditionally, women sit with the knees together while men separate them slightly, proportional to body size. Some martial arts, notably kendo and iaido, may prescribe up to two fist widths of distance between the knees. The big toes may rest side by side or are sometimes overlapped.

The act of stepping into and out of seiza is intended to be mindfully performed. There are codified traditional methods of entering and exiting the sitting position depending on occasion and type of clothing worn.

Seiza is most often done on tatami (woven straw mat) floors, but many people sit seiza-style on carpeted and even hardwood floors. Depending on the formality of the occasion, the setting, and the relative status of the person, it is sometimes acceptable to sit on a special cushion called a zabuton (座布団, literally a "sitting futon").

Sitting cross-legged is considered informal and is inappropriate for certain situations, but is sometimes permitted, especially for those for whom seiza is difficult, such as elderly or non-Japanese people (though in the case of the latter it is advisable, particularly in formal situations, to at least try). Sometimes stools are provided for elderly or injured people even when others are expected to sit seiza-style.

Doing seiza is an integral and required part of several traditional Japanese arts, such as tea ceremony, meditation (Zazen), and certain martial arts. Seiza is also the traditional way of sitting while doing other arts such as shodo (calligraphy) and ikebana (flower arranging), though with the increasing use of western-style furniture it is not always necessary for these nowadays.

Many theatres for traditional performing arts such as kabuki still have audience seating sections where the spectators sit in seiza style.

Special seiza stools are available in Japan. They are folding stools, small enough to be carried in a handbag, which are placed between the feet and on which one rests the buttocks when sitting seiza-style. They allow one to maintain the appearance of sitting seiza while discreetly taking pressure off the heels and feet.