Updated project metadata. The RLTPR cytosolic protein has an essential role in CD28 costimulation but its mode of action and importance in human T cells remain elusive. Here, using mass spectrometry we showed that CD28, RLTPR and the CARMA1 cytosolic adaptor physically associate in mouse T cells. Although RLTPR is thought to function as an actin-uncapping protein, this property was dispensable for CD28 costimulation. Our findings underpin the convergent function of RLTPR in T cells and suggest that its scaffolding role predominates during CD28 costimulation.