Ugliness is the property of physical things that are unappealing to the eye. The term is often used in reference to human appearance. The opposite of ugliness is beauty. Being a matter of subjective aesthetics, one person may think something beautiful that another may find ugly ("Beauty is in the eye of the beholder").

Although usually thought of in its visual sense, the property of ugliness may also be ascribed to other phenomena, for example, music, literature, scientific theories, human actions, and so on.