Sneathia amnii is a poorly characterized emerging pathogen that has been implicated in amnionitis and urethritis. We found that S. amnii damages fetal membranes, and we identified and purified a cytotoxic exotoxin that lyses human red blood cells and damages cells from fetal membranes. The gene appears to be cotranscribed with a second gene that encodes a protein with identity to two-partner system transporters, suggesting that it is the “A,” or secreted component of a type Vb system. The toxin is 1,881 amino acids with a molecular weight of approximately 200 kDa. It binds to red blood cell membranes and forms pores with a diameter of 2.0 to 3.0 nm, resulting in osmolysis. Because it appears to be the “A” or passenger component of a two-partner system, we propose to name this novel cytotoxin/hemolysin CptA for cytopathogenic toxin component A. IMPORTANCE Sneathia amnii is a very poorly characterized emerging pathogen that can affect pregnancy outcome and cause urethritis and other infections. To date, nothing is known about its virulence factors or pathogenesis. We have identified and isolated a cytotoxin, named CptA for cytopathogenic toxin, component A, that is produced by S. amnii. CptA is capable of permeabilizing chorionic trophoblasts and lysing human red blood cells and, thus, may play a role in virulence. Except for small domains conserved among two-partner secretion system passenger proteins, the cytotoxin exhibits little amino acid sequence homology to known toxins. In this study, we demonstrate the pore-forming activity of this novel toxin.