In drylands, microbes that colonise rock surfaces were linked to erosion because water scarcity excludes traditional weathering mechanisms. We studied the origin and role of rock biofilms in geomorphic processes of hard lime and dolomitic rocks that feature comparable weathering morphologies though originating from arid and hyperarid environments, respectively. We hypothesised that weathering patterns are fashioned by salt erosion and mediated by the rock biofilms that originate from the adjacent soil and dust. We used a combination of microbial and geological techniques to characterise rocks morphologies and the origin and diversity of their biofilm. Deep sequencing suggested that bacterial diversity is low and dominated by Proteobacteria, Cyanobacteria and Actinobacteria. These phyla formed laminar biofilms only on rocks exposed to atmospheric conditions and burrowed up to 6 mm beneath the surface, protected by sedimentary deposits. Unexpectedly, the biofilm composition was distinct from the communities identified in the settled dust and adjacent soil. Moreover, the rock bacterial communities were shown to secrete extracellular polymeric substances that mitigate desiccation, reducing water loss by over 40%. We therefore propose that microbial colonisation of mineral surfaces protect the rocks from weathering by retaining the moisture provided by dew from rapid desiccation thus mitigating salt crystallisation pressure within rocks’ pores attenuating stress erosion. Concomitantly, the biofilm layer stabilises the rock surface via coating protecting the weathered front. Our hypothesis contradicts common models, which considered biofilms as weathering-promoting agents and propose their role as mitigators of geomorphic processes in hot, arid environments.