Another cause of an armpit rash that’s not an infection is psoriasis, a chronic autoimmune condition. One form of psoriasis is especially likely to show up on skin folds, including under your arm.

Armpit rashes can appear in different ways, such as bumpy, smooth, or dry. Causes range from a heat rash to health conditions like psoriasis. Treatment will depend on the cause. Some armpit rashes ...

where people get psoriasis in their armpits or in their genital area. And when it is in a warm moist environment, that can lead to increased risk of skin infection.