We assessed species composition and relative biomass densities of large medusae (bell diameters larger than 4 cm) at four locations in Puget Sound, Washington, over two sampling periods (June and September). We specifically sampled sites in southern Hood Canal (near Hoodsport), northern Hood Canal (Hazel Point), southern Admiralty Inlet (Useless Bay) and Possession Sound using a bottom trawl as part of a larger survey of demersal fish and invertebrate community structure. In total, four species were observed during June sampling: three schyphomedusae (Phacellophora camtschatica, Cyanea capillata, Aurelia labiata) and one hydromedusa (Aequorea victoria). The highest jellyfish densities in June were observed off Hoodsport where Aurelia labiata and Cyanea capillata dominated the assemblage. In the remaining locations, Cyanea capillata dominated catches. In September, relative biomass densities were lower throughout the area surveyed and species composition simplified with usually one species of jellyfish clearly dominating each location. Our results indicate that jellyfish biomass changed markedly within and among locations, which has implications for modeling energy flows in Puget Sound and developing monitoring schemes that are able to capture interannual variability in jellyfish biomass. Given the abundance of jellyfish in our survey and their potential as sentinels of change in the marine environment we recommend that jellyfish populations be routinely monitored in Puget Sound.