The water-proton signal, overwhelmingly considered a nuisance in nuclear magnetic resonance spectroscopy, is advantageously used as a tool to assess protein concentration and to detect protein aggregates in aqueous solutions. The protocols in this article describe use of the water-proton transverse relaxation rate to determine concentration and aggregate content in protein solutions. Detailed recommendations and description of the parameter settings and data processing ensure successful implementation of this technique, even by a user with limited experience in magnetic resonance relaxometry. All measurements are done noninvasively, in a sealed container, without sampling or otherwise aliquoting the solution. The magnetic resonance relaxometry approach offered in this article could be advantageous for analysis of biologics formulations or when use of conventional analytical techniques is not possible. © 2019 by John Wiley & Sons, Inc. Basic Protocol 1: Nuclear magnetic resonance (NMR) relaxometry to measure protein concentration Basic Protocol 2: NMR relaxometry to measure protein aggregation.