We provide a unifying foundation for monetary policy and macroprudential policies in financial markets for economies with nominal rigidities in goods and labor markets and constraints on monetary policy such as the zero lower bound or fixed exchange rates. Macroprudential interventions in financial markets are beneficial because of an aggregate demand externality. Ex post, the distribution of wealth across agents affects aggregate demand and output through Keynesian channels. However, ex ante, these effects are not privately internalized in the financial decisions agents make. We obtain a simple formula that characterizes the size and direction for optimal financial market interventions as a function of a small number of empirically measurable sufficient statistics. We also characterize optimal monetary policy. We then show how to extend our framework to also incorporate financial markets frictions giving rise to pecuniary externalities. Finally, we provide a number of relevant concrete applications of our general theory.