•Plants must rearrange the network of complex carbohydrates in their cell walls during normal growth and development. To accomplish this, all plants depend on proteins called expansins that non-enzymatically loosen noncovalent bonding between cellulose microfibrils. •Surprisingly, expansin genes have more recently been found in some bacteria and microbial eukaryotes, where their biological functions are largely unknown. •Here, we reconstruct a comprehensive phylogeny of microbial expansin genes. We find these genes in all eukaryotic microorganisms that have structural cell wall cellulose, suggesting expansins evolved in ancient marine microorganisms long before the evolution of land plants. We also find expansins in an unexpectedly high diversity of bacteria and fungi that do not have cellulosic cell walls. These bacteria and fungi inhabit varied ecological contexts mirroring the diversity of terrestrial and aquatic niches where plant and/or algal cellulosic cell walls are present. •The microbial expansin phylogeny shows evidence of multiple horizontal gene transfer events within and between bacterial and eukaryotic microbial lineages, which may in part underlie their unusually broad phylogenetic distribution. Overall, expansins are unexpectedly widespread in bacteria and eukaryotes, and the contribution of these genes to microbial ecological interactions with plants and algae has likely been underappreciated.