We report herein a biopolymer actuator with a modular design that allosterically transduces ligand binding into an aqueous demixing phase transition. The biopolymer actuator consists of two modular domains: a ligand binding protein domain, calmodulin (CaM), that is fused to a transducer domain, a stimulus-responsive elastin-like polypeptide (ELP) that exhibits a reversible lower critical solution temperature (LCST) phase transition. We demonstrate that binding of calcium to CaM spontaneously triggers the phase transition of the attached ELP, leading to formation of meso-microscale particles depending on the chain length of the ELP. This behavior is reversible as chelation of the bound calcium results in dissolution of the assembled particles, is selective for calcium as opposed to magnesium, and is abolished by the binding of a peptide ligand that is specific to calcium-bound CaM. These results are, to our knowledge, the first demonstration of biomolecular recognition-triggered, allosteric regulation of the LCST phase transition of a polymer and are significant because they expand the available triggers of the LCST transition of stimulus-responsive polymers to biochemical ligand binding. The ability to allosterically trigger the LCST transition of ELPs by biomolecular recognition will be useful for developing "smart" polymer actuators that capitalize upon the myriad ligand-protein pairs that are available from biology and for application in the design of selective pull-down assays in proteomics, drug delivery, and nanoscale biomolecular devices.