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Abstract Macromolecules exhibit rich phase behavior that may be exploited for advanced material design. In particular, the volume phase transition in certain crosslinked hydrogels is a key property controlling the transition between a collapsed/dehydrated and a swollen/hydrated state, thereby regulating the release and absorption of water via a temperature change. In this work, a simple and tunable system exhibiting a carbon dioxide (CO₂)‐switchable volume phase transition is introduced, which displays isothermal swelling‐shrinking behavior that is activated by addition and removal of CO_2,respectively. Through systematic compositional studies, shifts in phase transition temperatures of up to 8.6 °C are measured upon CO₂exposure, which enables pronounced isothermal swelling in response to CO₂, reaching up to a fivefold increase in mass. The shift in transition temperature and the extent of swelling are controlled by the hydrogel composition, thus enabling the transition temperature and swelling degree to be tuned a priori for a particular application. Controlled release experiments from these gels upon a CO₂‐induced phase transition suggest viability for drug delivery applications. It is anticipated that this work will motivate and expand efforts to exploit phase behavior for smart material development.