Pgp3 is one of the products produced by a small (about 7.5 kb) but highly conserved cryptic plasmid contained in some chlamydiae. This protein is secreted into the cytoplasm of the host cells and known as a virulence factor associated with hydrosalpinx. Its native conformation is a triplex that can be recognized by host immune system. Pgp3 can neutralize the antichlamydial activity of human cathelicidin LL37. This review presents current research progress on the structure, pathogenesis and applications of Pgp3, and provides the new sights in chlamydial pathogenesis and vaccine preparation.