The adapter protein ADAP regulates T lymphocyte adhesion and activation. We present evidence for a previously unrecognized function for ADAP in regulating T cell receptor (TCR)-mediated activation of the transcription factor NF-κB. Stimulation of ADAP-deficient mouse T cells with antibodies to CD3 and CD28 resulted in impaired nuclear translocation of NF-κB, a reduced DNA binding, and delayed degradation and decreased phosphorylation of IκB (inhibitor of NF-κB). TCR-stimulated assembly of the CARMA1-BCL-10-MALT1 complex was substantially impaired in the absence of ADAP. We further identified a region of ADAP that is required for association with the CARMA1 adapter and NF-κB activation but is not required for ADAP-dependent regulation of adhesion. These findings provide new insights into ADAP function and the mechanism by which CARMA1 regulates NF-κB activation in T cells.