Smooth muscle is an involuntary non-striated muscle, found within the tunica media layer of large and small arteries and veins, the bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, arrector pili of skin, the ciliary muscle, and iris of the eye.

Anti-smooth muscle antibody are antibodies (immunoglobulins) formed against smooth muscle. These antibodies are typically associated with chronic autoimmune hepatitis.