A liquid supply unit includes a pressure adjusting valve portion that includes a liquid outlet connectable to an upstream end of a supply passage, which supplies liquid to a liquid ejection head, a liquid inlet that draws in the liquid from a liquid supply source containing the liquid, a pressure chamber at least partially formed by a flexible portion configured to be deformed when the pressure changes, an urging member configured to urge the flexible portion in a direction in which the flexible portion increases a volume of the pressure chamber, and an opening-closing valve configured to allow the liquid to be drawn from the liquid supply source to the pressure chamber through the liquid inlet in accordance with a deformation of the flexible portion.