A red spot on the armpit may be caused by florid chaff rash, contact dermatitis, prickly heat and other diseases. 1. Mottled furuncle: this disease is caused by Malassezia infection, but also related to heredity, high temperature and humid environment, malnutrition, excessive secretion of skin oil, excessive sweating and other factors. The main characteristics of lesions are light red, light brown, etc., which can occur in armpits, forehead and other places, so the patient’s armpit has a red spot may be caused by florid chaff rash. 2. Contact dermatitis: this disease is caused by skin contact with exogenous allergens, such as antiperspirant, detergent, soap, depilatory creams, etc., and then erythema, itching and other symptoms will appear in the contact area, which can be manifested as a erythema in the armpit. 3. Prickly heat: The causes of this disease include sweat duct obstruction, excessive environmental temperature, clothing impermeable, bacteria attached to the surface of the skin, etc. Prickly heat mostly occurs in the armpits, forehead, etc., so the patient will have a red spot on the armpit. It is recommended that patients with a red spot on the armpit should go to the hospital to clarify the specific cause and cooperate with the doctor for active treatment.