Postoperative delirium is a common postoperative complication in older patients, and its pathogenesis and biomarkers remain largely undetermined. The gut microbiota has been shown to regulate brain function, and therefore, it is vital to explore the association between gut microbiota and postoperative delirium. Of 220 patients (65 years old or older) who had a knee replacement, hip replacement, or laminectomy under general or spinal anesthesia, 86 participants were included in the data analysis. The incidence (primary outcome) and severity of postoperative delirium were assessed for two days. Fecal swabs were collected from participants immediately after surgery. The 16S rRNA gene sequencing was used to assess gut microbiota. Principal component analyses along with a literature review were used to identify plausible gut microbiota, and three gut bacteria were further studied for their associations with postoperative delirium. Of the 86 participants [age 71.0 (69.0–76.0, 25–75% percentile of quartile), 53% female], 10 (12%) developed postoperative delirium. Postoperative gut bacteriaParabacteroides distasoniswas associated with postoperative delirium after adjusting for age and sex (Odds Ratio [OR] 2.13, 95% Confidence Interval (CI): 1.09–4.17,P= 0.026). The association between delirium and bothPrevotella(OR: 0.59, 95% CI: 0.33–1.04,P= 0.067) andCollinsella(OR: 0.57, 95% CI: 0.27–1.24,P= 0.158) did not meet statistical significance. These findings suggest that there may be an association between postoperative gut microbiota, specificallyParabacteroides distasonis, and postoperative delirium. However, further research is needed to confirm these findings and better understand the gut-brain axis’s role in postoperative outcomes.