The Canon EOS 40D is a 10.1-megapixel semi-professional digital single-lens reflex camera. It was initially announced on 20 August 2007 and was released at the end of that month. It is the successor of the Canon EOS 30D, and is succeeded by the EOS 50D. It can accept EF and EF-S lenses. Like its predecessor, it uses an APS-C sized image sensor, resulting in a 1.6x field of view crop factor.

Canon EOS is an autofocus single-lens reflex camera (SLR) and mirrorless camera series produced by Canon Inc. Introduced in 1987 with the Canon EOS 650, all EOS cameras used 35 mm film until October 1996 when the EOS IX was released using the new and short-lived APS film. In 2000, the D30 was announced, as the first digital SLR designed and produced entirely by Canon. Since 2005, all newly announced EOS cameras have used digital image sensors rather than film. The EOS line is still in production as Canon's current digital SLR (DSLR) range, and, with the 2012 introduction of the Canon EOS M, Canon's mirrorless interchangeable-lens camera (MILC) system. In 2018 the system was further extended with the introduction of the EOS R camera, Canon's first full frame mirrorless interchangeable lens system.