We investigate the nature of information flow in turbulence from an information-thermodynamic viewpoint. For the fully developed three-dimensional fluid turbulence described by the fluctuating Navier-Stokes equation, we prove that information of large-scale eddies is transferred to small scales along with the energy cascade. We numerically illustrate our findings using a shell model and further show that in the inertial range, the intensity of the information flow is nearly constant and can be scaled by the large-eddy turnover time. Our numerical results also suggest that the corresponding information-thermodynamic efficiency is quite low compared to other typical information processing systems such as Maxwell's demon. These findings provide another perspective on how universality and intermittency of turbulent fluctuations emerge at small scales. Published by the American Physical Society 2024