31-class patrol boats (第三十一号型哨戒艇,, Dai Sanjūichi Gō-gata Shōkaitei) were a class of patrol boats of the Imperial Japanese Navy (IJN), serving during World War II.

In 1939, the IJN was liberated from London Naval Treaty, and they built many Kagerō-class destroyers.

Their boilers were degraded due to the operating conditions present in destroyers.

Some Momi-class destroyers were not able to show 30 knots speed.

The Navy General Staff made the Confidential Document No.