We analyze the effect of monetary policy on mergers and acquisitions (M&A) activity using transaction and balance sheet data of public U.S. companies. We provide causal evidence that contractionary monetary policy reduces M&A activity. The response is particularly pronounced among financially constrained and higher-marginal product of capital firms, causing a change in the composition of firms conducting M&A. This in turn reduces the efficiency at which capital is reallocated via M&A.