BACKGROUND We examined the effects of short bouts of structured physical activity (SBS‐PA) implemented within the classroom setting as part of designated gross‐motor playtime on preschoolers PA. METHODS Preschools were randomized to SBS‐PA (centers, N = 5; participants, N = 141) or unstructured free playtime (UPA) (centers, N = 5; participants, N = 150). SBS‐PA consisted of structured PA implemented in the classroom during the first 10 minutes of gross‐motor playtime followed by 20 minutes of free playtime. UPA consisted of 30 minutes of unstructured free playtime. Teachers implemented both conditions for 5 days/week for 6 months. PA was assessed with accelerometers (preschool‐day) and direct observation (30‐minute sessions). Generalized linear mixed models were used to examine the impact of the intervention. RESULTS Regarding the 30‐minute sessions, significant group main effects were observed for intervals spent at light (p < .001) and moderate‐to‐vigorous PA (MVPA, p < .001). Regarding the preschool‐day PA, significant group by visit interaction was observed for percent time spent in total preschool‐day MVPA (F (2, 254) = 3.54, p = .03). Percent of time spent in MVPA significantly decreased in both groups at 3 months and at 6 months. CONCLUSION SBS‐PA can be implemented in classroom settings; however, further research is needed to examine its impact on preschoolers PA levels.