There are many reasons for rib pain under the armpit, the common ones are intercostal neuritis, herpes zoster and trauma. 1. Intercostal neuritis: inflammation of the intercostal nerves caused by various reasons, manifested as pain in the area innervated by the intercostal nerves, which is often characterized by pinprick and burning pain. 2. Herpes zoster: an infectious disease caused by varicella zoster virus, which can affect the nerves and skin, manifesting as pain, blisters distributed in groups along the peripheral nerves, usually not crossing the midline of the body, and can occur in the chest, under the armpits and other parts of the body. 3. Trauma: soft tissue injury under the armpit or rib fracture caused by indirect or direct violence can also cause pain and swelling in the damaged area. Rib pain under the armpit may have other causes such as multiple myeloma, etc. It is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and targeted treatment under the guidance of the doctor.