Many infectious agents have the need to inhibit host apoptosis, and various strategies are known. Chlamydia trachomatis is an obligate intracellular bacterium replicating in a vacuole in the human cytosol. Chlamydia-infected human cells are strongly protected against apoptosis. We here mapped this anti-apoptotic activity to the direct blockade of the effectors of mitochondrial apoptosis, the Bcl-2-family proteins Bax and Bak. Molecular analysis of Bak revealed the inhibition of activation-associated conformational changes by the infection. Surprisingly, we identified the C. trachomatis outer membrane porin OmpA in a complex with Bak on mitochondria. When expressed in uninfected human cells, OmpA was imported into mitochondrial membranes. OmpA alone blocked apoptosis and reproduced the changes to Bak-activation observed during infection, reminiscent of the established activity of the human mitochondrial porin, VDAC2. The results suggest that C. trachomatis utilizes the relationship between gram-negative bacteria and mitochondria to block apoptosis and to secure its intracellular growth.