<p>Controlling heat flow at the microscale with transistor-like precision has long been a challenge, as thermal energy in solids typically diffuses without an efficient gating mechanism. Hydrodynamic electron fluids, such as the Dirac fluid in charge neutral graphene, offer a distinct approach by supporting non-diffusive, wave-like heat transport that could overcome this limitation. Here we demonstrate a graphene based thermal transistor that uses electrostatic gating to actively modulate these entropy-carrying heat waves—so called demon modes—in an ultraclean graphene channel. A local gate creates a narrow carrier density barrier that can either transmit or reflect the propagating entropy wave, yielding on/ off heat flow modulation exceeding 80%. On-chip time-resolved terahertz microscopy is used to directly visualize the wave propagation and its gate-controlled switching behavior. Furthermore, two-fluid hydrodynamic simulations quantitatively reproduce the observed transmission characteristics and reveal that the switching is governed by impedance matching between the gated and ungated regions of the electron fluid. Our results establish a foundation for active thermal circuitry and on-chip heat logic based on hydrodynamic electron transport.</p>