Brackish-water fishes are fish that live in brackish waters of lakes, sponges, estuaries and desalinated areas of seas, but avoid the open sea. They are subdivided into semi-passage fish, which enter rivers for reproduction (omul, vobla), and brackish-water fish proper (gobies).

Brackish waters are natural waters with salinity values between 0.5 g and 30 g/kg or 0.5% to 30% ppm.