Biofilm production in Pseudomonas aeruginosa is regulated by the intracellular secondary messenger cyclic diguanylate (c-di-GMP). C-di-GMP is produced by diguanylate cyclases (DGCs) and degraded by phosphodiesterases (PDEs). C-di-GMP controls motility, biofilm formation and virulence factor production in many bacteria. While much is known about how DGCs and PDEs synthesize and degrade c-di-GMP, respectively, little is understood about how environmental stimuli modulate enzyme activity. We identified the thermosensing diguanylate cyclase (tdcA) which allows for the thermoregulation of biofilm formation in P. aeruginosa. Activity assays revealed that TdcA activity increased ~35 fold as the temperature of the system increased from environmental to body temperature. Further, analysis revealed that TdcA acts as a thermostat, allowing for TdcA activity to be modulated by temperature. Our results demonstrate that TdcA is a DGC that acts as a molecular thermostat in P. aeruginosa.