Materials that can be switched between low and high thermal conductivity states would advance the control and conversion of thermal energy. Employing in situ time-domain thermoreflectance (TDTR) and in situ synchrotron X-ray scattering, we report a reversible, light-responsive azobenzene polymer that switches between high (0.35 W m ⁻¹ K ⁻¹ ) and low thermal conductivity (0.10 W m ⁻¹ K ⁻¹ ) states. This threefold change in the thermal conductivity is achieved by modulation of chain alignment resulted from the conformational transition between planar ( trans ) and nonplanar ( cis ) azobenzene groups under UV and green light illumination. This conformational transition leads to changes in the π-π stacking geometry and drives the crystal-to-liquid transition, which is fully reversible and occurs on a time scale of tens of seconds at room temperature. This result demonstrates an effective control of the thermophysical properties of polymers by modulating interchain π-π networks by light.