1. Despite recent advances in understanding community assembly processes, appreciating how these processes vary across multiple spatial scales and environmental gradients remains a crucial issue in ecology. 2. This study aimed to disentangle the drivers of diversity and composition of seaweed communities through a gradient of spatial scales based on a hierarchical sampling design consisting of 19 sites distributed in four sectors along the Brittany coastline. Using randomised community matrices and Moran’s eigenvector maps (MEMs), we compared i) the relative importance of deterministic and stochastic processes, ii) the environmental correlates of community composition and iii) the scale of variation in community composition for seaweed communities located at two different tidal heights. 3. Processes shaping community patterns are expected to vary along a gradient of tidal heights. Therefore, we specifically examined the following hypotheses: the contribution of deterministic over stochastic processes as well as the relative importance of environmental filtering over biotic interactions should be enhanced for seaweed communities of the infralittoral fringe compared to subtidal ones, whereas dispersal of propagules in the water column should be more restricted resulting in finer-scale variation in community composition for seaweed communities of the infralittoral fringe compared to subtidal communities. 4. Seaweed communities were largely shaped by deterministic processes, although the relative importance of deterministic processes was greater for communities of the infralittoral fringe than for subtidal communities. Sea surface temperature and geophysical variables were correlates of community composition at the two tidal heights; additionally, waves and current were correlated with the composition of the communities of the infralittoral fringe while kelp density was correlated with the composition of subtidal communities. Variation in community composition was observed at a finer scale for infralittoral fringe than for subtidal communities. Synthesis. Our results suggest that the relative importance of deterministic and stochastic processes in structuring seaweed communities varies across tidal heights. Furthermore, the MEMs framework highlights that the nature of environmental correlates and the spatial scale at which they were good correlates of community composition also vary across tidal heights and may therefore be useful to broaden our understanding of community assembly across vertical gradients.