Tyrosine phosphorylation of adaptor proteins permits the B cell antigen receptor (BCR)-associated protein tyrosine kinases to regulate downstream effector molecules. Here, we report the identification of a novel B cell adaptor for phosphoinositide 3-kinase (PI3K), termed BCAP. Tyrosine phosphorylation of BCAP is mediated by Syk and Btk, thereby providing binding site(s) for the p85 subunit of PI3K. Disruption of the BCAP gene in the DT40 B cell line inhibits BCR-mediated phosphatidylinositol 3,4,5-trisphosphate generation, leading to impaired Akt response. Moreover, recruitment of PI3K to glycolipid-enriched microdomains (GEMs) is significantly attenuated in the absence of BCAP. Hence, these data suggest that BCAP bridges BCR-associated kinases to the PI3K pathway by regulating PI3K localization.