T cell receptor (TCR) signaling is initiated by Src-family kinases (SFKs). To understand how C-terminal Src kinase (Csk), the negative regulator of SFKs, controls the basal state and the initiation of TCR signaling, we generated mice expressing a PP1-analog inhibitor-sensitive Csk allele (CskAS). Inhibition of CskAS in thymocytes, without TCR engagement, induced potent SFK activation and proximal TCR signaling up to phospholipase C gamma-1 (PLCγ1). Surprisingly, increases in inositol phosphates (IP), intracellular calcium and ERK phosphorylation were impaired. Altering the actin cytoskeleton pharmacologically or providing CD28 costimulation rescues these defects. Thus, Csk plays a critical role in preventing TCR signaling. However, our studies also reveal a requirement for actin remodeling, initiated by costimulation, for full TCR signaling.