Significance Molecular reporters that “remember” the activity history of a cell and give a corresponding signal that can be read out by imaging, sequencing, or other modalities are valuable tools for the study of neural networks. Here, we develop a reporter that drives stable expression of any transgene (such as a fluorescent protein) in response to cellular calcium activity during a user-defined time window. As calcium is a universal proxy for neural activity, our reporter provides a stable “snapshot” of neural activity which can be used for subsequent imaging, analysis, or manipulation of behavior-associated neural networks. A core feature of our reporter is an engineered calcium-activated protease, which cuts a protein sequence to reveal a transcription factor when cellular calcium levels rise.