A strap-on dildo (also simply a strap-on) is a dildo designed to be worn, usually with a harness, during sexual activity. Harnesses and dildos are made in a wide variety of styles, with variations in how the harness fits the wearer, how the dildo attaches to the harness, as well as various features intended to facilitate stimulation of the wearer or a sexual partner. Strap-on dildos can be used by people of any gender or sexuality.

A strap-on dildo can be used for a wide variety of sexual activities, including vaginal sex, anal sex, pegging, oral sex, or masturbation. Sexual lubricant can be used to ease insertion.