Purpose: To understand the prevalence and potential health effect of prolonged workplace sedentary (sitting) time, valid measures are required. Here, we examined the criterion validity of a brief self-reported measure of workplace sitting time and breaks in sitting time. Methods: An interviewer-administered questionnaire was used to assess workplace sitting time (h•d) and breaks from sitting per hour at work in a convenience sample of 121 full-time workers (36% men, mean age = 37 yr, 53% office based). These self-reported measures were compared with accelerometer-derived sedentary time (hours per day, <100 counts per minute) and breaks per sedentary hour (number of times,≥100 counts per minute) during work hours. Results: Self-reported sitting time was significantly correlated with accelerometer-derived sedentary time (Pearson r = 0.39, 95% confidence interval = 0.22-0.53), with an average sitting time 0.45 h•d -1 higher than average sedentary time. Bland-Altman plots and regression analysis showed positive associations between the difference in sitting and sedentary time and the average of sitting and sedentary time (mean difference =-2.75 h + 0.47 × average sitting and sedentary time; limits of agreement = ±2.25 h•d -1). The correlation of self-reported breaks per sitting hour with accelerometer-derived breaks per sedentary hour was also statistically significant (Spearman r s = 0.26, 95% confidence interval = 0.11-0.44). Conclusions: This study is the first to examine the criterion validity of an interviewer- administered questionnaire measure of workplace sitting time and breaks in sitting time using objective criterion measures. The workplace sitting measure has acceptable properties for use in observational studies concerned with sedentary behavior in groups of workers; however, the wide limits of agreement suggest caution in estimating individuals' sitting time with high precision. Using self-reported measures to capture patterns of workplace sitting (such as breaks in sitting time) requires further development.