Plenum may refer to:
- Plenum chamber, a chamber intended to contain air, gas, or liquid at positive pressure
- Plenism, or Horror vacui ("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
- Plenum filtration, aquarium filtration using a deep sand bed with a gap below it