Sedentary behaviour tracks from early to middle childhood, suggesting the need to intervene early. The aim of this systematic review was to identify determinants of change in accelerometer-assessed sedentary behaviour in young children, with a view to informing interventions. Ten electronic databases were searched. Longitudinal and intervention studies were included if they (a) targeted sedentary behaviour in young children (less than of equal to 6 years), (b) assessed change in accelerometer-assessed sedentary behaviour, and (c) reported on at least one determinant of change in sedentary behaviour. Intervention components were coded according to clusters of behaviour change technique (BCT) (ie, grouping similar BCTs components). Data synthesis was guided by the socioecological model. Sixteen studies (four longitudinal; 12 intervention) met the inclusion criteria. Two (out of five identified determinants) were associated with an increase in sedentary behaviour in longitudinal studies: the after childcare/school period and transition from childcare to school. Three (out of 21 identified determinants) were associated with a decrease in sedentary behaviour in intervention studies: "goals and planning" (ie, "behavioural contract"), "repetition and substitution" (ie, "graded tasks"), and "reward and treat" (ie, "incentives"). The environmental and interpersonal determinants identified in this review may help to inform behavioural strategies, timing, and settings for future interventions.