Pimples on the buttocks are actually relatively common. Dirt and moisture get trapped by your clothing -- add the friction that comes from sitting, and you've got a recipe for breakouts. Technically, however, pimples on the buttocks aren't considered acne; usually you're experiencing one of two related conditions. Folliculitis is a surface inflammation of the hair follicles; cysts or boils, also known as nodulocystic acne, are deeper and larger bumps under the skin. Treatment options are similar to those for true acne, however.
Wash your buttocks at least once daily in a hot shower. Heat and moisture encourage your pores to open up and clear out, so this step is particularly valuable if you have pustules that seem ready to burst. Soap up a washcloth to combine gentle exfoliation with cleaning; exfoliating your skin also opens up the pores and helps to both clear up any acne that's already present and prevent future breakouts. Use a loofah sponge for more heavy-duty exfoliation if needed, but take care not to irritate the skin. Choose an antibacterial soap if your skin can tolerate it, or a mild castile soap if your skin is sensitive.
Sit on a hot towel for 20 minutes. Repeat three to four times a day. Like a hot shower, the heat of the towel encourages the pores to open up and can also help cysts and boils to rise to the surface of your skin and begin to drain.
Apply benzoyl-peroxide ointment to your buttocks once a day. It's best if you apply a thin film to the entire area rather than just to problem spots. Benzoyl peroxide is a strong anti-acne treatment with an antibacterial action; its main side effect is that it can dry out skin. If benzoyl peroxide is too strong, you can try a product that contains salicylic acid. The best time to perform this step is just before bedtime. Regular application should clear up any acne within five to seven days.