Black spots on the arm may be caused by nevus, flat warts, hyperpigmentation, etc. Patients should consult a doctor in time to clarify the cause of the disease and then treat it. 1. Nevus: A black spot on the arm may be a nevus, which is formed by the benign proliferation of melanocytes, and has clear boundaries, regular edges, and uniform coloration. 2. flat warts: black spots on the arm may also be caused by human papilloma virus infection caused by flat warts, some patients may have mild itching, more than a sudden appearance, the number of and dense. 3. Hyperpigmentation: black spots on the arm may also be caused by hyperpigmentation, mostly seen in inflammatory skin diseases after healing. There may be other reasons for black spots on the arm, it is recommended to go to the hospital, a clear diagnosis. The symptoms should be treated accordingly.