Dark armpits may be caused by acanthosis nigricans, local inflammatory stimulation and other reasons. 1. Acanthosis nigricans: Acanthosis nigricans may be related to obesity, heredity and other factors, occurring in the armpits, neck and other skin folds, manifested as hyperpigmentation of the skin surface, the color of the initial grey, brownish-yellow, and then deepen into black, and at the same time, the skin can be obviously dry, rough or accompanied by papillary thickening and other manifestations. 2. Local inflammatory stimulation: If the patient’s armpit has suffered from certain inflammatory diseases, such as fungal infection, dermatitis, etc., under the repeated stimulation of inflammation, it will lead to the activation of the local melanocyte function, which will lead to the phenomenon of armpit darkening. Dark armpits may also have other causes, it is recommended to go to the hospital in a timely manner to improve the examination, clear cause to give targeted treatment.