CD22 (Siglec‐2) is a B‐cell membrane‐bound lectin that recognizes glycan ligands containing α2,6‐linked sialic acid (α2,6Sia) and negatively regulates signaling through the B‐cell Ag receptor (BCR). Although CD22 has been investigated extensively, its precise function remains unclear due to acting multiple phases. Here, we demonstrate that CD22 is efficiently activated in trans by complexes of Ag and soluble IgM (sIgM) due to the presence of glycan ligands on sIgM. This result strongly suggests sIgM as a natural trans ligand for CD22. Also, CD22 appears to serve as a receptor for sIgM, which induces a negative feedback loop for B‐cell activation similar to the Fc receptor for IgG (FcγRIIB).