AbstractStomata are a pivotal adaptation of land plants and control gas exchange. While most plants present solitary stomata, some plant species experiencing chronic water deficiency display clustered stomata on their epidermis; for instance, limestone‐grown begonias. Moreover, the membrane receptor TOO MANY MOUTHS (TMM) plays a major role in spacing stomata on the epidermis inArabidopsis, but the function of itsBegoniaorthologs is unknown. We used two Asian begonias,Begonia formosana(single stomata) andB. hernandioides(clustered stomata), to explore the physiological function of stomatal clustering. We also introduced theBegonia TMMs intoArabidopsis tmmmutants to study the function ofBegoniaTMMs.B. hernandioidesshowed higher water use efficiency under high light intensity, smaller stomata, and faster pore opening thanB. formosana. The short distance between stomata in a cluster may facilitate cell‐to‐cell interactions to achieve synchronicity in stomatal movement.BegoniaTMMs function similarly toArabidopsisTMM to inhibit stomatal formation, although complementation by TMM from the clustered species was only partial. Stomatal clustering in begonias may represent a developmental strategy to build small and closer stomata to achieve fast responses to light which provides tight support between stomatal development and environmental adaption.