Most quantum information measures are defined for quantum states. For example, mutual information measures the correlation between two subsystems in a quantum state. It is natural to ask whether correlation in spacetime can be characterized by some generalization of mutual information. In this work, we propose a space-time generalization of mutual information. The key idea is to consider a general "quantum experiment" that measures the correlation between two space-time regions, and use the setup of hypothesis testing. We discuss various properties of the spacetime mutual information, including how it provides an upper bound for all connected correlation functions, which is a direct generalization of the similar property of ordinary mutual information.