Water‐compatibility of heterogeneous photocatalysts has been pursued for energy and environmental applications. However, there exists a trade‐off between hydrophilicity and recyclability of the photocatalyst. Herein, we report a conjugated polymer photocatalyst with tertiary amine terminals that reversibly binds CO2 in water, generating a switchable hydrophilicity. The CO2‐assisted hydrophilicity boosted up the photocatalytic efficiency in aqueous medium with minimum dosage. When CO2 was desorbed, the photocatalyst could be simply regenerated from reaction media, facilitating the repeated use of photocatalyst. The hydrophilicity/hydrophobicity control of the polymer photocatalyst has successfully showcased through a variety of organic photo‐redox reactions under visible light irradiation in water.