One strategy to reduce soot formation in compression-ignition engines is extending the ignition delay to provide more time for mixing. However, vapor-fuel concentration measurements have shown that near-injector mixtures become too lean to achieve complete combustion, leading to a relative increase in unburned hydrocarbon emissions. One potential contributor to over-leaning is an "entrainment wave,"which is a transient increase in local entrainment after the end of injection. Although an entrainment wave can be predicted by a one-dimensional (1D) free-jet model, no previous measurements at diesel injection conditions have demonstrated conclusively its existence, nor has its magnitude been verified. Using particle image velocimetry (PIV) in the ambient gases, we measure entrained gas velocity through a diesel jet boundary before, during, and after the injection. The entrainment calculation depends on the definition of the jet boundary, here newly proposed based on the minimum of the radial coordinate and the radial velocity (r upsilon(r\)). Unlike previous formulations, the method is robust even in the presence of axial flow gradients in the ambient gases. Prior to the end of injection, the measured entrainment rates that agree well with non-reacting steady gas-jet behavior, as well as with the 1D free-jet model. After end of injection, the local entrainment rate temporarily increases by a factor of 2, which is similar to the factor 2.5 increase predicted by the 1D model. However, the entrainment wave is more broadly distributed in the experimental data, likely due to confinement and/or other real-jet processes absent in the 1D model.