T-follicular helper (T-FH) cells are a subset of CD4(+) helper T cells that help germinal center (GC) B-cell differentiation and high-affinity antibody production during germinal center reactions. Whether important extracellular molecules control T-FH differentiation is not fully understood. Here, we demonstrate that a secreted protein extracellular matrix protein 1 (ECM1) is critical for T-FH differentiation and antibody response. A lack of ECM1 inhibited T-FH cell development and impaired GC B-cell reactions and antigen-specific antibody production in an antigen-immunized mouse model. ECM1 was induced by IL-6 and IL-21 in T-FH cells, promoting T-FH differentiation by down-regulating the level of STAT5 phosphorylation and up-regulating Bcl6 expression. Furthermore, injection of recombinant ECM1 protein into mice infected with PR8 influenza virus promoted protective immune responses effectively, by enhancing T-FH differentiation and neutralizing antibody production. Collectively, our data identify ECM1 as a soluble protein to promote T-FH cell differentiation and antibody production.