NaK is an alloy of sodium and potassium, and particularly one that is liquid at room temperatures. It is a commercially available material in various grades. Alloys with between about 40% and 90% potassium by weight are liquid at room temperatures.

Eutectic NaK is 78% potassium and 22% sodium. It is liquid from -12.6 degrees celsius to 785 degrees celsius.

One notable use is as the coolant in experimental fast reactors. Unlike commercial plants, these are frequently shut down and defuelled. Use of lead or sodium, the other materials used in practical reactors, would require continual heating to maintain the coolant as a liquid. Use of NaK overcomes this. NaK is used in many other heat transfer applications for similar reasons.

NaK is also used as a catalyst for many reactions, including precursors of ibuprofen.

Nak is highly reactive with air or water, and must be handled with special precautions. Quantities as small as one gram can be a fire or explosion risk.