In microbial biofilms, bacterial cells are encased in a self-produced matrix of polymers (e.g., exopolysaccharides) that enable surface adherence and protect against environmental stressors. For example, the wrinkly spreader phenotype ofPseudomonas fluorescenscolonizes food/water sources and human tissue to form robust biofilms that can spread across surfaces. This biofilm largely consists of bacterial cellulose produced by the cellulose synthase proteins encoded by thewss(WS structural) operon, which also occurs in other species, including pathogenicAchromobacterspecies. Although phenotypic mutant analysis of thewssFGHIgenes has previously shown that they are responsible for acetylation of bacterial cellulose, their specific roles remain unknown and distinct from the recently identified cellulose phosphoethanolamine modification found in other species. Here, we have purified the C-terminal soluble form of WssI fromP. fluorescensandAchromobacter insuavisand demonstrated acetylesterase activity with chromogenic substrates. The kinetic parameters (kcat/KMvalues of 13 and 8.0 M−1s−1, respectively) indicate that these enzymes are up to four times more catalytically efficient than the closest characterized homolog, AlgJ from the alginate synthase. Unlike AlgJ and its cognate alginate polymer, WssI also demonstrated acetyltransferase activity onto cellulose oligomers (e.g., cellotetraose to cellohexaose) with multiple acetyl donor substrates (p-nitrophenyl acetate, 4-methylumbelliferyl acetate, and acetyl-CoA). Finally, a high-throughput screen identified three low micromolar WssI inhibitors that may be useful for chemically interrogating cellulose acetylation and biofilm formation.