South-East Asia is a biodiversity hot spot for several different animal and plant taxa, and grapsoid crabs are dominant components of its mangrove macrofauna. However, autecological traits of the species and assemblage structures are still largely undescribed. During the period 2012–14, we surveyed six mainland and insular mangrove sites along the west coast of the Malay Peninsula, four of which had not been investigated previously. Species composition differed among sites with different types of substrate and forest area. Small islands, characterised by coarser intertidal substrates, hosted remarkably different assemblages from mainland systems. Most of the species occurred in a small number of sites, suggesting stenotopic ecological traits or patchy distributions, and a marked variation in species composition and environmental conditions among sites was observed. This suggests that management actions assuming that this region’s coastal wetlands have comparable community compositions may likely lead to local extinctions of species, possibly affecting the regional biodiversity of these systems.