During placentation, the cytotrophoblast differentiates into the villous cytotrophoblast and the extravillous cytotrophoblast. The latter invades the decidualized endometrium. Glycodelin-A (GdA) is abundantly synthesized by the decidua but not the trophoblast. Previous data indicate that GdA suppresses the invasion of trophoblast cell lines by down-regulating proteinase expression and activities. This study addresses the signaling pathway involved in the above phenomenon. GdA was found to suppress phosphorylation of ERKs and expression of their downstream effector c-Jun, a component of the transcription factor activator protein-1 (AP-1). The involvement of ERKs and c-Jun in suppressing trophoblast invasion and biosynthesis of proteinases was confirmed by using siRNA knockdown and pharmacological inhibitors. Desialylation reduced binding affinity of GdA toward and invasion suppressive activities on the trophoblast. Co-immunoprecipitation showed that Siglec-6 on the trophoblast was the binding protein of GdA. The binding of GdA to Siglec-6 was sialic acid-dependent. Treatment with anti-Siglec-6 antibody abolished the invasion suppressive activities of GdA. These results show that GdA interacts with Siglec-6 to suppress trophoblast invasiveness by down-regulating the ERK/c-Jun signaling pathway.