Propionibacterium acnes is a relatively slow growing, typically aerotolerant anaerobic gram positive bacterium that is linked to the skin condition acne; it can also cause chronic blepharitis and endophthalmitis, the latter particularly following intraocular surgery. The genome of the bacterium has been sequenced and a study has shown several genes that can generate enzymes for degrading skin and proteins that may be immunogenic (activate the immune system).

This bacteria is largely commensal and thus present on most people's skin; and lives on fatty acids in the sebaceous glands on sebum secreted by pores. It may also be found throughout the gastrointestinal tract in humans and many other animals. It is named after its ability to generate propionic acid.