Background <p>Periodontitis, a progressive inflammatory disease of the tooth-supporting structures, has direct adverse effects on systemic health. Links between periodontitis and gastrointestinal tract malignancies have also emerged in numerous studies. The aim of our study was to uncover the prevalence of periodontitis and its effects on postoperative complications among patients undergoing gastric surgery.</p> Methods <p>This prospective study took place between May 2017 and June 2019 at the Helsinki University Hospital, with data from patients undergoing gastric surgery. We recorded complications based on the Clavien-Dindo classification, and for periodontitis screening we used the aMMP-8 point-of-care test.</p> Results <p>Our study included 59 patients, of whom 29 (49%) tested positive for periodontitis. Malignant tumour histology was present in 75% of cases. Periodontitis showed no association with postoperative complications (<i>p</i> = 0.879): neither with surgical site infections (<i>p</i> = 0.149) nor with thrombotic events (<i>p</i> = 0.513).</p> Conclusions <p>The prevalence of periodontitis among patients undergoing gastric surgery matched that of the general population in Finland. Periodontitis did not associate with postoperative complications in surgical patients.</p> Trial registration <p>ClinicalTrials.gov, NCT03144973. Date of registration: 09/05/2017.</p>