We demonstrate the passthrough of Treasury supply to deposit funding through bank market power. We show that an increase in Treasury supply leads to a net deposit outflow. At the same time, reliance on wholesale funding decreases. The effect is heterogeneous|banks in more competitive markets experience stronger deposit out-flows. The explanatory power of Treasury supply is not driven by monetary policy and bank-specific investment opportunities. We rationalize our empiricalfindings with a model of imperfect deposit competition. Consistent with Drechsler, Savov and Schnabl (2017), the model and empirical evidence predict the opposite effect for monetary policy rate hikes: there is a larger response in less competitive markets.