The SH2B family has three members (SH2B1, SH2B2 and SH2B3) that contain conserved dimerization (DD), pleckstrin homology (PH) and Src homology 2 (SH2) domains. The DD domain mediates the formation of homo- and heterdimers between members of the SH2B family. The SH2 domain of SH2B1 (previously named SH2-B) or SH2B2 (previously named APS) binds to phosphorylated tyrosines in a variety of tyrosine kinases, including JAK2 and the insulin receptor, thereby promoting the activation of JAK2 or the insulin receptor, respectively. JAK2 binds to various members of the cytokine receptor family, including receptors for growth hormone and leptin, to mediate cytokine responses. In mice, SH2B1 regulates energy and glucose homeostasis by enhancing leptin and insulin sensitivity. In this work, we identify SH2B2 as a new isoform of SH2B2 (designated as SH2B2) derived from the SH2B2 gene by alternative mRNA splicing. SH2B2 has a DD and PH domain but lacks a SH2 domain. SH2B2 bound to both SH2B1 and SH2B2, as demonstrated by both the interaction of glutathione S-transferase-SH2B2 fusion protein with SH2B1 or SH2B2 in vitro and coimmunoprecipitation of SH2B2 with SH2B1 or SH2B2 in intact cells. SH2B2 markedly attenuated the ability of SH2B1 to promote JAK2 activation and subsequent tyrosine phosphorylation of IRS1 by JAK2. SH2B2 also significantly inhibited SH2B1- or SH2B2-promoted insulin signaling, including insulin-stimulated tyrosine phosphorylation of IRS1. These data suggest that SH2B2 is an endogenous inhibitor of SH2B1 and/or SH2B2, negatively regulating insulin signaling and/or JAK2-mediated cellular responses.