Cross-sectional study. To evaluate the reliability and validity of the Thai version of the Swiss Spinal Stenosis (SSS) questionnaire. The SSS questionnaire was developed to assess symptom severity, physical functional status, and patient satisfaction in spinal stenosis patients that was previously reported to have good reliability and validity. There is currently no Thai version of the SSS questionnaire. The SSS questionnaire was translated into Thai language to create the Thai version of the Swiss Spinal Stenosis (Thai-SSS) questionnaire. Translation was performed according to international standards using a forward-backward translation protocol. Translation was performed by two expert translators and one physician, and the final version was approved by an expert committee. Thai patients with degenerative lumbar spinal stenosis were enrolled and evaluated using the Thai-SSS questionnaire, the Thai version of the Short Form-36 (SF-36), and a visual analogue scale (VAS) before and after treatment. Reliability and validity of the Thai-SSS were assessed via comparison with the SF-36 and the VAS. One hundred seven patients were included. Eighty patients underwent decompression and fusion surgery, and the others underwent decompression alone. Cronbach's alpha of all domains of the Thai-SSS showed excellent internal consistency (0.869–0.921). The intraclass correlation coefficient for test–retest reliability was 0.77 (95% CI: 0.65–0.85). Regarding concurrent validity, the physical function scale of the Thai-SSS was strongly correlated with the physical functioning domain of the SF-36 (r = 0.70). The symptom severity domain of the Thai-SSS was strongly correlated with the bodily pain domain of the SF-36 and the VAS (r = 0.76 and 0.78, respectively). The Thai-SSS was found to be a valid and reliable tool for evaluating symptom severity, physical function, and patient satisfaction after treatment in Thai patients with spinal stenosis. Level of Evidence: 4.