Technical Abstract: Campylobacter is one of the leading causes of bacterial human acute gastroenteritis. These microorganisms are highly prevalent in poultry semen and may contribute to vertical transmission between the breeder hen and offspring. Unfortunately, strategies to reduce or eliminate these pathogens in poultry semen negatively impact sperm viability. Many plant essential oils have been reported to exhibit antimicrobial activity against bacteria, fungi and viruses. The objective of our study was to examine the efficacy of trans-cinnamaldehyde, the main ingredient of cinnamon, to reduce Campylobacter concentrations in chicken semen. Semen was collected from roosters, pooled and diluted with semen extender, then divided into treatments: negative control (no Campylobacter, no Trans-cinnamaldehyde), treatments containing serial dilutions of a 0.42% trans-cinnamaldehyde stock solution (1:1 through 1:16 dilutions; 0.24 to 0.015% of stock solution) inoculated with a wild-type Campylobacter jejuni and held at 4 or 23C for 2 h. Semen was stored at 4C for an additional 24 h and assessed for Campylobacter concentrations and sperm viability at 2, 6 and 24 h utilizing SYBR 14/Propidium iodide live/dead stain and fluorescent microscopy. The study was replicated eight times. After 2h at 23C a 2 log reduction in Campylobacter was observed in the 0.12 and 0.24% trans-cinnamaldehyde treatment groups compared to controls. In the 4C treatments, no differences were observed between treatments and controls after 2 h however Campylobacter was not detectable in the 0.12 and 0.24% trans-cinnamaldehyde treatment groups and significantly lower in all but the lowest trans-cinnamaldehyde treatment group compared to control after 24 h in vitro storage. Sperm viability remained at 80% or above for all groups. Trans-cinnamaldehyde reduced Campylobacter in semen, without detrimentally effecting sperm viability and might provide a practical solution to eliminate Campylobacter in poultry semen after in vitro storage.