Signal transducing adaptor protein-2 (STAP-2) is a recently identified adaptor protein that contains Pleckstrin and Src homology 2 (SH2)-like domains as well as a YXXQ motif in its C-terminal region. STAP-2 is also known as breast tumor kinase (Brk) substrate (BKS). Our previous studies revealed that STAP-2 binds to signal transducer and activator of transcription 3 (STAT3) and STAT5, and regulates the signaling pathways downstream of them. In the present study, we identified tyrosine-250 (Tyr250) in STAP-2 as a major site of phosphorylation by Brk, using a series of STAP-2 YF mutants and anti-phospho-STAP-2 Tyr250 antibody. Furthermore, overexpression of the STAP-2 Y250F mutant protein affected Brk-mediated STAT3 activation. Importantly, small-interfering RNA-mediated reduction of endogenous STAP-2 expression decreased Brk-mediated STAT3 activation. Taken together, our findings demonstrate that STAP-2 is phosphorylated at Tyr250 by Brk, and plays an important role in Brk-mediated STAT3 activation.