Plenum may refer to:

• Plenum chamber, a chamber intended to contain air, gas, or liquid at positive pressure
• Plenism, or Horror vacui (physics) the concept that "nature abhors a vacuum"
• Plenum (meeting), a meeting of a deliberative assembly in which all members are present; contrast with quorum
• Plenum space, enclosed spaces (in buildings) used for airflow
• Plenum cable, electrical wire permitted in plenum spaces per building codes
• Plenum Publishing Corporation, a publisher of scientific books and journals
• Plenum (physics), a space completely filled with matter
• Undergravel filters, in aquarium filtration, an open space under a layer of gravel or sand
• Air-mixing plenum, a place where ducts meet