A full-face diving mask is a type of diving mask that seals the whole of the diver's face from the water and contains a mouthpiece, demand valve or constant flow gas supply that provides the diver with breathing gas. The full face mask has several functions: it lets the diver see clearly underwater, it provides the diver's face with some protection from cold and polluted water and from stings, such as from jellyfish or coral. It increases breathing security and provides a space for equipment that lets the diver communicate with the surface support team.

Full-face masks can be more secure than breathing from an independent mouthpiece; if the diver becomes unconscious or suffers an oxygen toxicity convulsion, the diver can continue to breathe from the mask, unlike a scuba mouthpiece which is normally gripped between the teeth.

Full-face diving masks are often used in professional diving. They are relatively rarely used in recreational diving.

The full-face mask provides breathing gas to the diver, and an air space to facilitate underwater vision. There is usually one air space for both breathing and vision, and this is automatically equalised by the breathing gas during descent. However, some models have a mouthpiece isolating the breathing gas supply from the rest of the interior of the mask, or have a divided interior, and the main air space must then be equalised during descent by exhaling through the nose.

Most full-face masks have an open circuit demand gas supply, but free-flow and closed circuit applications also exist and models used in commercial diving may be normally demand supplied, but with a free-flow option selectable by the diver.

For some applications it is desirable for the gas inside the mask or helmet to remain at a pressure slightly above ambient at all times while in the water, as this will prevent any contamination from leaking into the gas space during inhalation if the face or neck seal, or the exhaust valve system, does not seal perfectly. In clean water such a leak is a minor problem, but leaks of contaminated water can be a hazard to health, and even life-threatening. A positive pressure inside a free-flow helmet is easily achieved by slightly increasing the opening pressure of the exhaust valve, provided it is adjustable, but for a demand system the cracking pressure of the demand valve must also be adjusted, so that it delivers gas before the internal pressure drops below external ambient pressure. This is not difficult, as a slight adjustment to second stage valve spring pressure is all that is required. The problem is that when the mask or helmet is off the diver, and the gas supply is pressurised, the demand valve will leak continuously, and a large amount of gas can be lost. The Interspiro Divator Mk II mask has a second stage regulator which has a manual lock on the demand valve to prevent free-flow when the mask is not in use, which unlocks when a breath is taken, and must be reset when the mask is taken off.

A full-face mask consists of a transparent faceplate (lenses, or window) through which the diver can see, a connection to a source of breathing gas, a means of removing any water which may get inside, some facility for the diver to block the nose to equalise the ears, a means of sealing these components to the diver's face and preventing ingress of water and a means of securing the assembly to the diver's head. Additional components may include communications equipment, lights, alternative breathing gas connections, and a means to defog the interior surface of the faceplate.

Two basic configurations are in common use: Those with a soft elastomer skirt which seals around the periphery of the face, and those with a rigid frame supporting the other components, which is sealed to the face by a soft foam ring which is clamped to the frame by a metal band, to which the straps are secured (band mask)^{[citation needed]}