There are many reasons why the red spots on the arms are itchy, and the common ones are contact dermatitis, eczema, keratosis pilaris and other reasons. 1. Contact dermatitis: arm skin contact with external sensitizing or irritating substances, may cause allergic reaction or irritation reaction at the contact site, resulting in itchy red spots on the arm. 2. Eczema: Inflammatory skin reactions caused by internal and external stimuli, which may also manifest as itchy red spots on the arms. 3. Keratosis pilaris: mostly related to heredity, vitamin A deficiency and metabolic disorders, can be manifested as arm follicular keratotic papules, skin color, reddish brown, can be accompanied by itching. Red spots on the arm are very itchy, there may be other reasons, you need to go to the hospital in time to check and clarify the cause of the disease.