The process pp → W^-c produces polarized charm quarks. We suggest that ATLAS and CMS can measure this polarization in Run 2 by using Λ_c decays. This can become the first measurement ever of a longitudinal polarization of charm quarks. Its results will provide a unique input to the understanding of polarization transfer in fragmentation. They will also allow applying the same measurement technique to other (e.g., new physics) samples of charm quarks in which the polarization is unknown a priori. The proposed analysis is similar to the ATLAS and CMS measurements of the W+c cross section in the 7 TeV run that used reconstructed D-meson decays for charm tagging.