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Climate change modifies the abundance and distribution of marine species, which can reshape patterns of species richness. The Northeast US Continental Shelf (NES) is a mid-latitude marine ecosystem experiencing changes in its physical environment and biota; these changes involve both lower and upper trophic level organisms. In this study, change in species richness of fish and macroinvertebrates was examined based on trawl survey data. Using a constrained subset of the survey strata comprising the overall design, we observed some 451 species over the period 1968–2022. Species richness was consistently higher in the autumn survey versus the spring survey. This seasonal difference in richness was mainly due to a contrast in vertebrate taxa as invertebrate species richness was similar between the seasons. Significant trends were found in the species richness when considering all taxa in both spring and autumn surveys. The rate of change in species richness reflected an increase of 10.8 species per decade in spring and an increase of 16.5 species per decade in autumn. The enhanced rate of increase in autumn was reflected in taxonomic and functional groups that we examined, and likely resulted from longer summering phases by migratory vertebrate species and range shifts northward by multiple taxa in response to greater summer temperatures and longer summer duration. Species richness in the NES was positively correlated with temperature over the study period; however, richness was also positively correlated with ecosystem biomass, suggesting the response in species richness is not limited to the redistribution of species alone. We expect richness to continue to increase, especially in autumn, but range contractions and further community restructuring could lead to declines in richness in the northern end of the NES.