Understanding how community assembly processes vary across spatial scales and environmental gradients is key to predicting species coexistence and informing conservation priorities. Beta-diversity represents the spatial variation in biodiversity and is intimately linked to processes ranging from fully niche-based to neutral assembly processes. Here we investigated the beta-diversity patterns of rocky subtidal macrobenthic assemblages in two environmentally contrasting fjords of Chilean Patagonia and across three spatial scales and dimensions within each fjord: vertical scale (subtidal depth; 0 – 21 m), fine horizontal (within fjord sections; 1 - 10 km) and broad horizontal (between fjord sections; 10 - 100 km). We applied generalized dissimilarity models (GDMs) to analyse three underlying processes that shape ecological communities: density-independent responses to abiotic conditions (salinity and temperature), dispersal (geographical distances and depth), and stochasticity (beta-null deviations) on beta-diversity expressed as Bray-Curtis dissimilarities. Stochastic and dispersal processes dominated beta-diversity along horizontal gradients, particularly at larger spatial scales, while environmental filters consistently contributed to beta-diversity along depth gradients. The southern fjord presented higher community variation in line with higher environmental heterogeneity. Stochastic processes dominate the northern fjord, suggesting a greater influence of habitat homogenization. These results emphasize the scale-dependent nature of assembly processes in fjord ecosystems and underscore the importance of incorporating multiple spatial dimensions into biodiversity assessments.