Zalma, once known as "Bollinger's Mill", is an unincorporated community and census-designated place on the banks of the winding Castor River at a point where the river makes a horseshoe bend in southern Bollinger County in southeast Missouri, United States. When the railroad moved to town, the name Bollinger's Mill was changed to Zalma, after railroad worker Zalma Block. Zalma is also said to come from a Native American word meaning "the end." As of the 2020 U.S. Census, Zalma had a population of 73.