Interactive visualization of scientific data in a web browser can be a powerful tool, particularly in a context of open data sharing following the FAIR principles. It obviates the need to install visualization software locally, and allows scientists to explore shared datasets of potential interest before further analysis or download. NeoViewer is an open-source tool for visualizing and exploring neurophysiology datasets stored in a wide variety of file formats. It enables neuroscientists to view recordings of membrane potential, local-field potentials, spike trains, and other neurophysiological signals within their web browser. The tool consists of a web API, easily deployed as a Docker container, and a Javascript component, available for the ReactJS and AngularJS frameworks, that can be embedded in any web page.