Campylobacter causes human foodborne illness, and epidemiological evidence indicates poultry and poultry products as a significant source of human infection. Decreasing Campylobacter in the poultry intestinal tract would decrease contamination of poultry products. Caprylic acid is a medium-chain fatty acid reported to be effective in killing a variety of bacterial pathogens, including Campylobacter jejuni, but its effect has not been investigated in the control of C. jejuni in preslaughter market-aged poultry already colonized with this bacterium. The objective of this study was to determine the therapeutic effect of caprylic acid on C. jejuni counts in the cecal contents of 42-d-old chickens. Four trials were conducted. In the first 2 trials, day-of-hatch chicks (n = 60 per trial) were assigned to 6 treatment groups (n = 10 birds per treatment group): positive controls (Campylobacter, no caprylic acid), 0.7 or 1.4% of caprylic acid in feed for the last 3 d of the trial with or without a 12-h feed withdrawal. Treatments were similar for trials 3 and 4 except the doses used were 0.35 or 0.7% caprylic acid supplementation for the last 7 d of the trial. On d 42, ceca were collected and Campylobacter counts determined. The supplementation of caprylic acid at 0.35 and 0.7% consistently decreased (P < 0.05) the colonization of C. jejuni in the chicken ceca compared with positive control treatment. When these treatments were evaluated after a 12-h feed withdrawal period, 0.7% caprylic acid decreased Campylobacter colonization in the 3-d treatment supplementation. Body weight and feed consumption did not differ between the caprylic acid and control groups. The results suggest that therapeutic supplementation of caprylic acid in the feed can effectively decrease Campylobacter in market-aged chickens and may be a potential treatment for decreasing pathogen carriage in poultry.