The North Atlantic Deep Water is a water mass in the Atlantic Ocean. It is largely formed in the Labrador Sea where the North Atlantic drift ends in the northeast Atlantic (northeast of Iceland) by the sinking of dense overflow water from the Norwegian Sea. It runs around the southern end of Greenland and then follows the coast of Canada down to the coast of the United States where it turns a bit east, out from the coast and then continues southeast, past the eastern tip of South America.

The North Atlantic Deep Water is a cold stream that runs about 2-3 kilometers below the surface. The motion of it pulls the waters of the North Atlantic drift northward.

It has been shown that the North Atlantic Deep Water has shut down in the past (such as during the Younger Dryas), and that this decreases the strength of the Gulf Stream and the North Atlantic drift, in turn cooling the climate of northwestern Europe. There is concern that global warming might cause this to happen again.