Trichomonas tenax, an oral flagellated protozoon found in humans, potentially associated with the inflammation of periodontal tissues and decreased immunity that causes the tissue damage and tooth loss from chronic infection. Currently, there is a lack of data regarding the prevalence of T. tenax infection in Thailand. Therefore, this study aimed to measure prevalence of T. tenax in periodontal disease patients by using polymerase chain reaction (PCR) to amplify the 18S ribosomal RNA (18S rRNA) gene and to determine the factors associated with the presence of this protozoan. A cross-sectional descriptive study was conducted among 230 patients with periodontal disease, who visited the oral health center of Suranaree University of Technology Hospital, Thailand from 2021 to 2022. Dental plaque specimens were collected and examined to identify the presence of T. tenax using the PCR-based 18S rRNA gene. The occurrence of factors associated with T. tenax infection was analyzed by the chi-square test and binary logistic regression. The prevalence of T. tenax infection was 13.48% (31/230), in patients, including 96.77% (30/31) and 3.23% (1/31) in periodontitis and gingivitis patients, respectively. The presence of T. tenax was associated with periodontal disease (p<0.001) and the Periodontal Screening and Record (PSR) index (p=0.001). The significant risk factors for T. tenax infection were periodontitis (OR=239.89, 95% CI=23.801-2417.746), no-underlying disease (OR=0.31, 95% CI=0.099-0.942), and male sex (OR=0.25, 95% CI=0.062-0.981). Dentists should be concerned about this oral protozoan in periodontitis patients. Furthermore, epidemiologic studies of T. tenax are still needed to investigate the mechanism of pathogenesis from T. tenax infection.