Which sea has no coastline because it is defined by ocean currents?

The Sargasso Sea is the sea with no coastline because its boundaries are defined by ocean currents. It is a region of the North Atlantic rather than a sea enclosed by surrounding land.

Four major currents form its approximate boundaries: the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east, and the North Atlantic Equatorial Current to the south. Together, they create part of the clockwise-rotating North Atlantic Gyre.

The Sargasso Sea is named for Sargassum, a brown seaweed that floats in large mats at the surface. These mats provide shelter and feeding habitat for animals including young sea turtles, Sargassum fish, and other open-ocean species. The region is also important to the life cycles of American and European eels, which migrate there to spawn.

Its borders are not perfectly fixed: currents shift, and scientists may draw the region differently depending on the measurements used. That does not change the key distinction. The Sargasso Sea is widely recognized as the only named sea whose boundaries are set by water movement rather than land.