Locum, short for the Latin phrase locum tenens (lit. "place-holder," akin to lieutenant), is a person who temporarily fulfills the duties of another. For example, a Locum doctor is a doctor who works in the place of the regular doctor when that doctor is absent. These professionals are still governed by their respective regulatory bodies, despite the transient nature of their positions.

The abbreviated form "locum" is common in the United Kingdom, Canada, Australia, New Zealand, and most other countries; unlike in Latin its plural is locums. In the United States, the full length "locum tenens" is preferred, though for some particular roles, alternative expressions (e.g. "substitute teacher") may be more commonly used.

In the UK, the NHS on average has 3 500 locum doctors working in hospitals on any given day, with another 6,000 locum general practitioners. Many of these locum hospital doctors are supplied by private agencies through a national framework agreement that the NHS holds with 21 private agencies. NHS figures show that approximately 80% of hospital locum positions are filled by agencies on this framework. The remaining 20% are filled by agencies working outside of this agreement.

On the other hand, GP locums (Freelance GPs) mostly work independently from locum agencies either as self-employed or via Freelance GP Chambers based on the NASGP's Sessional GP Support Team (SGPST) model.

Professions where locums are common include: