The cytokine interleukin-21 (IL-21) is mainly produced from activated CD4⁺ T cells and natural killer T (NKT) cells. IL-21 enhances the proliferation and differentiation of T cells and B cells and also increases cytotoxicity of CD8⁺ T cells and NK cells through the IL-21 receptor and its downstream signaling molecules such as signal transducers and activator of transcription 3 (STAT3) and extracellular signal-regulated kinase 1/2 (ERK1/2). SH2 domain-containing tyrosine phosphatase (SHP-2) is ubiquitously expressed, including hematopoietic cells. SHP-2 has been implicated in the regulation of IL-6 and IL-3 signaling, but its function in IL-21 signaling has not been investigated. Therefore, we studied the role of SHP-2 in IL-21 signaling by SHP-2 overexpression and knockdown experiments. For the SHP-2 overexpression, we used 293T human embryonic kidney cells, in which the IL-21 receptor system were easily reconstituted and high amounts of exogenous SHP-2 were expressed by vector transfection. In 293T cells, overexpressed SHP-2 caused the increase in the degree of the IL-21-induced ERK1/2 activation. Subsequently, SHP-2 knockdown experiments were performed in the mouse pro-B cell line, BAF21RWT-1, which constitutively expresses human IL-21 receptor and proliferates in an IL-21-dependent manner. SHP-2 knockdown reduced the degree of the IL-21-induced ERK1/2 activation and suppressed cell proliferation. These results suggest that SHP-2 may augment the ERK1/2 activity and cell proliferation activity in IL-21 signaling. We propose that SHP-2 is involved in the IL-21-mediated ERK1/2 activation and cell proliferation.