Trimix is a breathing gas, consisting of oxygen, helium and nitrogen, and is often used during the deep phase of dives carried out using Technical diving techniques.

Our normal breathing gas, air, consists of about 21% oxygen, 78% nitrogen, and 1% of other gases. The primary purpose of adding helium to the breathing mix is to reduce the percentage of nitrogen and oxygen to below that of air to allow the gas mix to be breathed safely at greater depth. A lower proportion of nitrogen is required to reduce the intoxication that is referred to as nitrogen narcosis and other physiological effects of the gas at depth. Lowering the oxygen content is necessary as oxygen becomes toxic as pressure, due to depth, increases. Oxygen toxicity can cause convulsions, which if they occur underwater can lead to drowning.

The exact ratio of gases in the mix is calculated to minimize narcosis and the risk of oxygen toxicity for the maximum depth of the dive. Safe limits for mix of gases in trimix are generally accepted to be a maximum partial pressure of oxygen (PPO₂ - see Boyle's Law) of 1.0-1.2 atm and maximum equivalent air depth of 30 meters (100 ft). For example, a mix for a 100 metre (330 ft) dive might be 10% oxygen, 70% helium, 20% nitrogen.

Skills beyond those taught to basic SCUBA divers are required to safely dive trimix as the use of trimix usually assumes deeper or longer dives requiring decompression and possibly the use of several tanks on one dive containing various gas mixes to facilitate the decompression process. Training is offered by several diver training organizations.