This study tested the equivalence of concurrently collected measurements of physical activity in children ages 3-5 years from the System for Observing Play and Leisure Activity in Youth (SOPLAY) and accelerometers. Accelerometry-based physical activity measurements were collected from 109 preschool children across 18 sites, and SOPLAY was used to concurrently observe and measure physical activity. Equivalence tests, correlation analyses, and Bland-Altman plots were completed for the two instruments' measurements. Equivalence tests demonstrated that SOPLAY and accelerometers are not equivalent measures of physical activity in children ages 3-5 years. Based on these results, other accelerometer parameters, including epoch length and cut-points, should be utilized to further compare SOPLAY and accelerometry. Additionally, an adapted version of SOPLAY with additional movement categories should be considered and compared against accelerometry as a more sensitive instrument for measuring physical activity in children ages 3-5 years.