Purpose <p>This study investigated the reliability and validity of the five-repetition sit-to-stand (5xSTS) and 30-second chair sit-to-stand (30s-CST) tests in individuals after lumbar spinal surgery.</p> Methods <p>This prospective psychometric study included individuals who had undergone lumbar spinal surgery and were at least 6 weeks post-operatively. Participants performed the 30s-CST, 5xSTS, 10-meter walk test (10MWT), and timed up and go test (TUG), with 2-minute rest intervals between tests. One hour later, the 30s-CST and 5xSTS were repeated for retest analysis. Test–retest reliability and agreement were assessed using the intraclass correlation coefficient (ICC) and Bland–Altman analysis, respectively. ICC values were interpreted as follows: &gt;0.90 excellent, 0.75–0.90 good, 0.50–0.75 moderate, and &lt; 0.50 poor. Concurrent validity was evaluated using Pearson correlation coefficients, and standard error of measurement (SEM) and minimal detectable change (MDC95) were also calculated.</p> Results <p>A total of 128 individuals were assessed for eligibility; 18 were excluded, and 110 participants were included in the final analysis. The test–retest reliability was excellent for both the 30s-CST [ICC (2,1) = 0.958, 95% CI: 0.939–0.971] and the 5xSTS [ICC (2,1) = 0.946, 95% CI: 0.922–0.962]. The SEM and MDC95 values were 0.56 and 1.55 for the 30s-CST, and 0.52 and 1.44 for the 5xSTS, respectively. Bland–Altman analysis revealed good agreement between test and retest measurements for both tests, with narrow limits of agreement and minimal systematic bias. Furthermore, both the 30s-CST and 5xSTS exhibited strong concurrent validity, demonstrating significant correlations with functional mobility measures, including the 10MWT and TUG.</p> Conclusion <p>The 30s-CST and 5xSTS are suggested to be reliable and valid performance tests for individuals after lumbar spinal surgery.</p>