The eastern water bat or Sakhalin bat (Myotis petax) is a species of mouse-eared bat. It was for a long time considered to be a subspecies of Myotis daubentonii.
The Eastern water bat is a forest bat found in Russia (south and eastern Siberia, Transbaikalia, Primorye, and Sakhalin), northern China, Mongolia, Korea, and Japan (Hokkaido).