A substitute teacher is a person who teaches a school class when the regular teacher is absent or unavailable; e.g., because of illness, personal leave, maternal leave and so on. "Substitute teacher" (usually abbreviated as "sub") is the most commonly used phrase in the United States, Canada (except Ontario and New Brunswick), India and Ireland, while supply teacher is the most commonly used term in Great Britain and the Canadian provinces of Ontario and New Brunswick. The term cover teacher is also used in Great Britain. Common synonyms for substitute teacher include relief teacher or casual relief teacher (used in Australia and New Zealand) and "emergency teacher" (used in some parts of the United States). Other terms, such as "guest teacher", are also used by some schools or districts. Regional variants in terminology are common, such as the use of the term teacher teaching on call (TTOC) in the Canadian province of British Columbia and occasional in Ontario.

In the United States, substitute teachers find jobs by first completing the application and interview process from their local school district. Once approved, they will either be enrolled in an automated calling system or more currently, via a system that uses the internet to post available substitute teaching assignments. Substitutes can also find work by contacting private schools in their district. Most substitute teachers in the U.S. can be assigned to work in all academic subject areas as needed (except for long-term substituting assignments). The substitute is generally responsible for closely following and continuing with the lesson plans left by the teacher, and to maintain student order and productivity. As with locum tenens physicians, the idea is that continuity exists between the work done by the substitute and that done by the other professional. Substitute teachers work with the same students as the regular teacher does. Substitute teachers can often work in multiple schools within one district, as well as for multiple school districts.