A gas leak refers to a leak of natural gas or another gaseous product from a pipeline or other containment into any area where the gas should not be present. Gas leaks can be hazardous to health as well as the environment. Even a small leak into a building or other confined space may gradually build up an explosive or lethal concentration of gas. Leaks of natural gas and refrigerant gas into the atmosphere are especially harmful due to their global warming potential and ozone depletion potential.

Leaks of gases associated with industrial operations and equipment are also generally known as fugitive emissions. Natural gas leaks from fossil fuel extraction and use are known as fugitive gas emissions. Such unintended leaks should not be confused with similar intentional types of gas release, such as:

• gas venting emissions which are controlled releases, and often practised as a part of routine operations, or
• "emergency pressure releases" which are intended to prevent equipment damage and safeguard life.

Gas leaks should also not be confused with "gas seepage" from the earth or oceans - either natural or due to human activity.