The Signal Transducers and Activators of Transcription (STATs) are a family of transcription factors that play important roles in cytokine signaling. Following cytokine stimulation they are tyrosine phosphorylated by receptor-associated Janus kinases (JAKs). Phosphorylation promotes their dimerization and this confers their ability to bind DNA consensus sequences and stimulate gene expression. STAT3 is a member of this family and can be activated by a large number of cytokines, growth factors and hormones. It is also a target of oncogenic tyrosine kinases that commonly link STAT3 closely with cancer. One of these tyrosine kinases is called breast tumor kinase (Brk). Brk is a non-receptor tyrosine kinase that is expressed in more than 60% of breast tumors. We have shown that Brk activates STAT3 and induces transcriptional activation of STAT3. One of the genes induced by tyrosine phosphorylated STAT3 is a negative regulator of cytokine signaling, the suppressor of cytokine signaling 3 (SOCS3). SOCS3 is known to block signaling mediated by cytokine receptors in a classical feedback loop. We have found that SOCS3 is also induced in response to Brk and it is able to inhibit the ability of Brk to phosphorylate STAT3. The molecular mechanism by which SOCS3 suppresses Brk activity has been investigated in this study. SOCS3 has several functional domains, a kinase inhibitory region (KIR), followed by an extended SH2 subdomain (ESS), an SH2 domain, and the conserved C-terminal SOCS box. A link to proteosomal degradation was discovered with the association of the SOCS box to components of the E3 ubiquitin ligase complex. It is demonstrated that the primary inhibitory function of SOCS3 on Brk is mediated by the KIR domain. SOCS3 physically associates with Brk and this association is mainly mediated by SH2 domain in SOCS3 and tyrosine kinase domain in Brk. In addition, SOCS3 promotes Brk degradation and the SOCS box is necessary for this effect. These findings identify Brk as a target of SOCS3, and demonstrate the inhibitory mechanism relies on binding to Brk and effecting both kinase activity and protein degradation.