The main aim of this study was to assess the criterion validity of the Sedentary Behavior Questionnaire (SBQ) to measure SB in community-dwelling older adults using thigh-measured accelerometry as the criterion method. 801 participants (75.6 ± 6.1 years old, 57.6% females) provided valid thigh-based accelerometer data (activPAL/Axivity) and completed the SBQ. Criterion validity was assessed using Spearman's Rho coefficients. Bland--Altman plots, including 95% limits of agreement and Intraclass Correlation Coefficient (ICC), were used to assess the agreement between self-report and device-measured daily SB time. Strength of the association was examined using multiple linear regression. There was a weak correlation (Rho = 0.25, p < 0.001) between self-reported and device-based SB measures. The SBQ under-estimated daily SB time compared to accelerometry. Our results highlighted an overall weak-to-moderate correlation between measures, with significant differences between each country's version. Researchers should be cautious when using the SBQ to provide an estimation of SB time in older adults.