The threespine stickleback fish (Gasterosteus aculeatus aculeatus) are in marine, brackish and coastal freshwater habitats of the northern hemisphere. They are found in boreal and temperate regions of the northern hemisphere and in marine waters and lowland freshwater habitats in the Atlantic and Pacific basins.

In the Atlantic Ocean, threespine sticklebacks are distributed from the Iberian Peninsula through the British Isles to Iceland and southern Greenland, and south along the east coast of North America to Chesapeake Bay. Freshwater populations are found throughout most of this range, but do not go farther south than Maine, USA. Freshwater populations are also distributed along the coast of the Mediterranean and in inland waters across Eastern Europe to the Baltic Sea.

In the Pacific Ocean, threespine sticklebacks are found from Baja California, Mexico northward along the coast of North America, across the Bering Strait, and then along the coast of mainland Asia and Japan to the southwest coast of Korea. Marine and freshwater populations are found in Japan, but the limit of marine populations in Asia is unclear. Freshwater populations are restricted to coastal areas in both Asia and North America.

Biogeographic Regions: nearctic (Native ); palearctic (Native ); atlantic ocean (Native ); pacific ocean (Native )