We isolated and characterized 12 microsatellite loci for the North American medicinal leech, Macrobdella decora. Macrobdella decora is abundant in central and northern North American freshwater systems. The resulting microsatellite library, the first for any species of leech or any clitellate annelid, demonstrates the efficacy of this type of analysis on unexploited leech populations and should serve as a baseline for comparison with other species, notably the commercially important and threatened European medicinal leech, Hirudo medicinalis.