Aim: Kelp forests provide habitat and food that supports a high diversity of flora and fauna. While numerous studies have described macroinvertebrates associated with kelp blades, stipes and holdfasts, a key kelp forest microhabitat, epilithic understory algae, remains poorly studied. Here, we used a macroecological approach and artificial seaweed units (ASUs) to explore the effects of ocean climate, wave exposure and habitat complexity on understory algal associated macroinvertebrate assemblages within Laminaria hyperborea forests in the United Kingdom. Location: 9° latitudinal gradient along the north and west coasts of the United Kingdom. Methods: Replicate ASUs comprising four different habitat complexities were deployed under mature L. hyperborea at 2 sites (along a wave exposure gradient, separated by km) within each of 4 locations (separated by 100s km) nested within two regions (warm and cold, spanning 9° of latitude). After 5 months in situ, the ASUs were collected and macroinvertebrates were identified to species level and enumerated. Results: Habitat complexity and wave exposure both influenced macroinvertebrate assemblage structure, but results also showed clear effects of ocean climate, with macroinvertebrate assemblages differing between warm and cool regions, primarily driven by higher diversity and evenness in the warmer region and greater abundance in the cooler region. Main conclusions: Predicted warming and a shift to less complex turf-forming algal assemblages are likely to alter the structure of macroinvertebrate assemblages associated with understory algae, with potential implications for kelp forest food web dynamics.