We propose a supply-side channel for the transmission of monetary policy. We show that when high-markup firms have lower pass-throughs than low-markup firms, then positive demand shocks, such as monetary expansions, alleviate cross-sectional misallocation by reallocating resources to high-markup firms. Consequently, positive ?demand shocks? are accompanied by endogenous positive ?supply shocks? that raise productivity and lower inflation. We derive a tractable, four-equation model where monetary shocks generate hump-shaped productivity responses. In our calibration, the supply-side effect amplifies the total impact of monetary shocks on output by about 70%. We provide empirical evidence validating our model?s predictions using identified monetary shocks.