Dark spots on arm skin may be caused by sun spots, seborrheic keratosis, pigmented nevus and so on, and patients should consult doctors in time to clarify the causes. 1. Sun spots: dark spots on the arm skin may be caused by long-term ultraviolet radiation, such people should pay attention to sunscreen, avoid prolonged ultraviolet irradiation, generally no special treatment. 2. Seborrheic keratosis: also known as age spots, may be related to the aging of the skin, mostly seen in the elderly over 40 years of age, may appear in the arm round or ovoid, border clear brown rash, can be treated by laser or cryotherapy. 3. Pigmented nevus: black spots on arm skin may also be caused by pigmented nevus, the cause of which has not been clarified yet, and it may be a benign hyperplasia or misshapen tumor formed by melanocytes, which can be removed through surgery and other means. Arm skin black spots may also be other diseases, it is recommended that patients go to the regular hospital in a timely manner, after a clear diagnosis under the guidance of the doctor standardized treatment.