Synchrony is a multifaceted biological phenomenon exhibited at various levels of organization. Here, we introduce the concept of structural synchrony, defined as the coherence of structural variations in multispecies assemblages occurring at a certain spatiotemporal scale, and propose a direct measure of this type of synchrony. This approach was first tested on numeric simulations imitating various types of model community dynamics and then applied to a large data set on White Sea intertidal harpacticoids, covering a wide range of spatial (decimeters to 140 m) and temporal (days to 25 yr) scales. Scale-specific analysis revealed distinct patterns and allowed us to draw inferences about processes that drive community dynamics. Short-term (up to 1 mo) changes occurred independently or even anti-synchronically, possibly due to trophic-driven migrations. On a seasonal scale, a high degree of within-habitat synchrony occurred at time intervals of 2-3 mo or longer but dropped with increasing spatial distance; this pattern is most likely caused by seasonal reproductive dynamics of populations. At the interannual scale, external exposure, i.e. long-term changes in sediment composition, appeared to be the strongest synchronizing force for the harpacticoid community, while this factor was locally active and became most apparent at the within-habitat scale. We evaluate the advantages and limitations of our framework, highlighting its broad domain of application and anticipating potential extensions. We encourage ecologists to integrate structural synchrony analysis into their toolbox to quantitatively evaluate spatiotemporal community dynamics and to link these dynamics with underlying processes.