A strap-on dildo (also strap-on, dildo harness) 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.

A strap-on dildo can be used for a wide variety of sexual activities, including solo or mutual masturbation, as well as penetrative oral, anal, or vaginal sex. Sexual lubricant is often used to ease insertion.

Strap-on dildos can be used by people of any sex or gender, upon people of any sex or gender. A person who does not have a penis may use a strap-on, as may a person whose penis does not allow for penetration, whether due to size or function. A person who does not want to use their penis for penetration may instead use a strap-on; this may include some trans women. Trans men may use strap-on dildos in order to facilitate experiencing sexual activity as many biological males do. People who are genderqueer or enjoy gender-bending may use a strap-on during sex in order to enjoy related forms of gender expression. Strap-on dildos are also used in kink scenes: those involving forced feminization of a man may involve the use of a strap-on upon that man, while those involving orgasm denial may involve a man using a strap-on upon a partner, to limit the amount of sexual stimulation which that man receives.