A blowout is the uncontrolled release of a formation fluid, usually gas, from a well being drilled, typically for petroleum production. A blowout is caused when a combination of well control systems fail—primarily drilling mud hydrostatics and blow-out preventers (BOPs)—and formation pore pressure is greater than the wellbore pressure at depth. When such an incident occurs, formation fluids begin to flow into the wellbore and up the annulus and/or inside the drill pipe, and is commonly called a kick. If the well is not shut in, a kick can quickly escalate into a blowout when the formation fluids reach the surface, especially when the fluid is a gas, which rapidly expands as it flows up the wellbore and accelerates to near supersonic speeds. Blowouts can cause significant damage to drilling rigs, and injuries or fatalities to rig personnel.

Prior to the development of blow-out preventers, blowouts were common and were referred to as gushers.