Laizhou Bay, an estuary of the Yellow River, lies to the north of the Shandong Peninsula and is one of the three largest and most important bays of the Bohai Sea. It is characterized by a high level of primary productivity and is an important spawning, nursery, and feeding ground for many fish species in the Yellow Sea and Bohai Sea. Medusae play crucial roles, not only ecologically in the marine ecosystem but also economically, as commercial target species for some large scyphomedusae in Laizhou Bay. In the past few decades, the interacting impacts of climate change and human activities have changed the ecosystem of Laizhou Bay significantly, leading to regime shifts in biological organisms, including medusae. Recently, increasing concern has prompted studies on the ecological impacts of changes in the gelatinous species community of the estuary and coastal area. To explore the temporal and spatial distribution of the composition and abundance of medusa species in the bay, annual surveys were designed and carried out from May 2011 to April 2012 (excluding December 2011 and January and February 2012, because of the presence of sea ice). Based on the data collected from 147 samples collected over the course of 9 cruises, nearly thirty cnidarians including twenty-four hydromedusae, one siphonophore, and three large scyphomedusan jellyfish were identified. The species of hydromedusae and siphonophore were small, and were collected by being vertically hauled from the seafloor to the surface using a mesoplankton net (mesh size 160 μm). Of the smaller medusae, no common species were collected throughout the year. Species richness peaked in August, while the total abundance peaked in May. One or two dominant species contributed over 80% of the total monthly abundance. The dominant species varied among months, with Rathkea octopunctata dominating from March to May, Sugiura chengshanense and Euphysora bigelowi dominating in June, E. bigelowi dominating in July, Eirene menoni dominating in September, and E. menoni and Muggiaea atlantica dominating in October. In August, E. menoni and Obelia geniculata were abundant, but were distributed regionally in the central parts of the bay. Very few medusa species were collected in November. Multiple statistical analyses, including cluster analysis and BVSTEP modules conducted with Primer 6.0 software, were used to identify the best matches for environmental factors that affect the composition of the medusa assemblages. Based on the abundance of small medusae, cluster analysis recognized four seasonal and spatial assemblage groups. The first group, with R. octopunctata as its representative species, was collected at the runoff inlet of the bay in spring. The second group was collected in the western section of the river mouth or the central parts of the bay in summer. The second group's representative species were S. chengshanense and E. bigelowi. The third group also occurred in summer, but its location was very similar to the spring group, and it included E. menoni as its representative species. The fourth group appeared in autumn and was collected in the river mouth or the central part of the bay, and included two indicator species:M. atlantica and Proboscidactyla flavicirrata. According to the results of the BVSTEP modules analysis, the water temperature had a more significant effect on the seasonal and spatial distribution of the hydromedusae than salinity, pH, or dissolved oxygen. The three large scyphomedusan jellyfish were only caught by fishing methods that utilized a net with a 20 mm mesh size. They occurred only at a few inlet sites after June. Aurelia spp. peaked later in October. Rhopilema esculentum and Nemopilema nomurai were collected from June to August at only a few sites.