The impact of additional structured outdoor playtime on preschoolers'; physical activity (PA) level is unclear. The purpose of this pilot study was to explore the effects of increasing structured outdoor playtime on preschoolers'; PA levels. Eight full-day classrooms (n = 134 children) from two preschool programmes were randomised into a treatment (STRUCT, n = 4) or control (CON, n = 4) condition. Both groups received an additional 30 minutes of outdoor playtime three days per week for four weeks. The STRUCT intervention consisted of previously tested structured outdoor playtime activities/games. The CON intervention consisted of free outdoor playtime. Children were individually recruited (n = 75) for the objective assessment of PA levels. The PA levels of 67 children (age, 4.1 [plus or minus] 0.8 years; STRUCT, n = 38; CON, n = 29) were assessed at baseline and during week four for seven consecutive days using Actigraph accelerometers. Data were analysed using mixed-model analysis of variance. Time spent in vigorous PA significantly increased during the 30-minute intervention time for the STRUCT group compared with the CON group (group x time interaction: F(1, 36) = 4.91, p = 0.04). Compared with baseline, a significant increase was observed in the STRUCT group's time spent engaged in moderate-to-vigorous PA (MVPA) during the intervention time, but this increase was not significant compared with the CON group (baseline: STRUCT, 1.7 [plus or minus] 2.0 min; CON, 1.9 [plus or minus] 2.4 min; week four: STRUCT, 4.9 [plus or minus] 3.1 min; CON, 3.3 [plus or minus] 2.5 min). Compared with the CON group, the STRUCT group spent a significantly greater percentage of time engaged in MVPA and a significantly lower percentage of time engaged in sedentary activity during the preschool day. In conclusion, this study provides preliminary evidence that increasing preschoolers'; exposure to structured activities during outdoor playtime could lead to improvements in their PA levels. The present intervention could be used to help preschoolers meet the recommended levels of PA. (Contains 2 tables and 1 note.)