Shimpan (審判) are the umpires of a professional sumo bout. In a sumo tournament five shimpan sit around the ring to observe which wrestler wins the matchup. An initial decision is given by the gyoji, which is usually correct and no action is taken by the shimpan.

If one of the shimpan disagrees or is unsure about the decision then he raises his hand and the five of them climb into the ring, or dohyo to hold a mono-ii. A mono-ii (of the shimpan only) can in principle also be called by any of the four sumo wrestlers awaiting their bout around the ring, although it is an extremely rare occurrence.

During the mono-ii the five shimpan give their views on what happened. The gyoji is usually able to listen in but is not expected to take part unless invited to do so. The deliberations are further advised by two further shimpan in a video room, in communication with the chief shimpan via an audio link in his ear.

The result of the mono-ii can be to uphold the gyoji's decision (so-called gunbai-dori), reverse his decision (gunbai-sashichigai), or call a rematch (torinaoshi). The chief shimpan is responsible to make the announcement and a brief explanation to the wrestlers and spectators.

All the shimpan are oyakata, or members, of the Japan Sumo Association. When judging tournament bouts they wear formal Japanese dress, or hakama.