Objectives This study investigated physical activity (PA) and sedentary behaviour (SB) among preschool-aged children in Singapore and potential correlates at multiple levels of the socioecological model from in-school and out-of-school settings. Design A cross-sectional study using a mixed-methods approach. Participants Parent-child dyads from six preschools in Singapore. Methods PA and SB of children (n=72) were quantified using wrist-worn accelerometers for seven consecutive days. Three focus group discussions (FGDs) among 12 teachers explored diverse influences on children's activities, and System for Observing Play and Leisure Activity in Youth (SOPLAY) assessed PA environment and children's activity levels at preschools. Seventy-three parents completed questionnaires on home and neighbourhood factors influencing children's PA and SB. Descriptive analyses of quantitative data and thematic analysis of FGDs were performed. Results Based on accelerometry, children (4.4±1.1 years) spent a median of 7.8 (IQR 6.4-9.0) hours/day in SB, and 0.5 (0.3-0.8) hours/day in moderate-to-vigorous physical activity (MVPA). MVPA was similar throughout the week, and SB was slightly higher on non-school days. In preschools, SOPLAY showed more children engaging in MVPA outdoors (34.0%) than indoors (7.7%), and absence of portable active play equipment. FGDs revealed issues that could restrict active time at preschool, including academic requirements of the central curriculum and its local implementation. The teachers had varying knowledge about PA guidelines and perceived that the children were sufficiently active. In out-of-school settings, parents reported that their children rarely used outdoor facilities for active play and spent little time in active travel. Few children (23.5%) participated in extracurricular sports, but most (94.5%) reported watching screens for 1.5 (0.5-3.0) hours/day. Conclusion MVPA was low and SB was high in preschool-aged children in an urban Asian setting. We identified diverse in-school and out-of-school correlates of PA and SB that should be taken into account in health promotion strategies.