The term secondary containment refers to any temporary or emergency containment system for a product or substance that is already retained by a primary system.

The classic example of a secondary containment system is the waterproofing of spill pools built around large tanks in the oil industry. This creates a safety system that guards against environmental disasters caused by surface run-off of highly contaminating liquids in emergencies, with the potential to affect large areas of land and even reach the level of underground water.

By definition, a secondary containment system should never be needed and should only ever have to operate for a short period time (e.g. hours or days). As such, it does not require extreme durability in terms of impermeability to the retained substance, a requirement that would increase costs without any financial benefit.