Tools that accurately assess physical activity and sedentary behaviors have broad implications relative to understanding the association of adverse health outcomes and these behaviors. Given the ease of distribution and inexpensive nature of self-report tools, they are the most widely used means to assess human behavior in large-scale populations. The purpose of this study was to validate the ACT24 online self-report recall for measures of sedentary and active behavior against criterion measure. Participants of a larger study were asked to complete the ACT24 recall on a random day in three different weeks during which they were wearing the criterion device. A total of 16 recalls were completed that were used to assess ACT24 measures of sedentary, active, and MVPA behavior. Four different comparisons afforded this analysis: criterion sitting time to ACT24 sedentary time, criterion standing time to ACT24 active behavior, criterion stepping time to ACT24 active behavior, and criterion stepping of 3.0+METs to ACT24 MVPA. Results for the comparisons made between ACT24 sedentary time versus criterion sitting time and ACT24 active time to criterion active time showed little systematic differences at the group level, but the limits of agreement were relatively wide. The comparisons made between ACT24 active time to criterion stepping time and ACT24 MVPA to criterion stepping time at 3.0+ METs both showed a positive systematic difference. Increased incidence of physical activity was correlated with more difference between the measures, likely due to an underestimation of criterion active time measurement. These results are important in the preliminary validity analysis of ACT24 measures of active and sedentary time. Future directions include implementing validation protocols in larger and more diverse samples.