A diving instructor is a person who trains, and usually also assesses competence, of underwater divers. This includes freedivers, recreational divers including the subcategory technical divers, and professional divers which includes military, commercial, public safety and scientific divers.

Depending on the jurisdiction, there will generally be specific published codes of practice and guidelines for training, competence and registration of diving instructors, as they have a duty of care to their clients, and operate in an environment with intrinsic hazards which may be unfamiliar to the lay person. Training and assessment will generally follow a diver training standard, and may use a diver training manual as source material.

Recreational diving instructors are usually registered members of one or more recreational diver certification agencies, and are generally registered to train and assess divers against specified certification standards. Originally these standards were at the discretion of each training and certification agency, but inter-agency and international standards now exist to ensure that the basic skills required for acceptable safety are included as a minimum standard for both instructors and recreational divers. Military diving instructors are generally members of the armed force for which they train personnel. Commercial diving instructors may be required to register with national government appointed organisations, and comply with specific training and assessment standards, but there may be other requirements in some parts of the world.

A diving instructor trains and assesses the competence of persons who intend to become underwater divers. The recreational diving instructor may use training standards and materials provided by the certifying organisation, or may develop their own suitable training materials, depending on the training system and authority involved. Most recreational training and certification agencies provide suitable training materials and require their registered instructors to train and assess using the materials provided.

Commercial diving schools may have a broader scope of options, and may be expected to develop and maintain their own training materials, which may be required to comply with a published training standard.

The earliest known diving manual was written by the Deane brothers in 1836. This described the use of the Deane helmet, which was not attached to the suit, and was open to the water at the bottom, and would flood if the diver bent over or fell, but could not cause helmet squeeze. Siebe Gorman produced diving manuals in the 19th century. By 1883 the Siebe Gorman manual was available in Spanish: Siebe y Gorman (1883). Manual Del Buzo E Instrucciones Para Operaciones Submarinas. Madrid: Imprenta De Fortanet. pp. 64 pages and 16 plates.

In 1953, Jack Atkinson, the first National Diving Officer of the British Sub-Aqua Club, and Colin McLeod started a system of training and accreditation for Third Class, Second Class and First Class Divers, which was based on the three-tier system with which they were familiar, as used for divers in the UK armed forces. This programme was developed over the next six years culminating in the first BSAC Diving Manual, published in 1959. Oscar Gugen, one of BSAC's founders, and George Brookes travelled the UK helping to set up branches of BSAC, all of which used amateur instructors to deliver the BSAC training programme within the club setting.

In 1955, Dottie Frazier became the world's first female scuba diving instructor. Dottie Frazier is also recognized as the first female hard-hat diver, first female instructor and the first female to own and operate her own dive shop. Along with diving, Frazier also worked with the United States Navy to design wet suits and dry suits for military and recreational divers.^{[citation needed]}