A swim diaper or pool diaper is a diaper that is made for infants and toddlers to be worn underneath a bathing suit. Swim diapers serve the purpose of protecting both the bathing suit and the pool water from the child's excrement.

Both reusable and disposable swim diapers exist. One of the most popular brands of reusable swim diapers is the SOSecure Containment Swim Brief, made by Discovery Trekking Outfitters. One of the most popular brands of disposable swim diapers is Little Swimmers, made by Huggies. They are sold in three sizes: small (16–26 lb or 7–12 kg), medium (24–34 lb or 11–15 kg) and large (over 32 lb or 14 kg+). They are not as absorbent as any of the other available Huggies brand products. This is due to the water-logged environment they are designed to be used in. They are designed mainly for easy clean-up after a swim. Pampers also make a disposable swim diaper called Pampers Splashers, which fit the same sizes as the Little Swimmers. They feature Go Diego Go for boys and Dora the Explorer for girls. One of the most popular brands of reusable swim diapers is Swimmies, made by Bummis. They are available in three sizes, small (7-13 lbs.), medium (13-20 lbs.), and large (20-28 lbs.) and a variety of bold prints.