We quantify the impact of bank market power on monetary policy transmission through banks to borrowers. We estimate a dynamic banking model in which monetary policy affects imperfectly competitive banks' funding costs. Banks optimize the pass-through of these costs to borrowers and depositors, while facing capital and reserve regulation. We find that bank market power explains much of the transmission of monetary policy to borrowers, with an effect comparable to that of bank capital regulation. When the federal funds rate falls below 0.9%, market power interacts with bank capital regulation to produce a reversal of the effect of monetary policy.WE EXAMINE THE QUANTITATIVE IMPACT of bank market power on the transmission of monetary policy through the banking system. This transmission channel is potentially important, given three decades of consolidation in the banking industry that has softened competitive pressure. Moreover,