BACKGROUND: Little is known on how relocations of time spent in different movement behaviours during pre-school hours could relate to preschooler’s fundamental movement skills (FMS), a key predictor of later physical activity (PA). Thus, the aim of this study was to examine whether the school-time composition was associated with FMS; and to investigate predicted differences in FMS when a fixed duration of time was reallocated from one activity behaviour to another in preschool children. METHODS: A cross-sectional representative data of an intervention study with Brazilian low-income preschoolers. Two hundred and four preschoolers of both sexes (4.5±0.8 years-old; 101boys) provided 10 hours of school-time objectively assessed PA and sedentary behaviour (SB) data (Actigraph wGT3X), and FMS assessments (TGMD-2). Association of school-time composition of movement behaviuors with FMS and its reallocations during school-time was explored using compositional analysis in R (version 1.40-1), robCompositions (version 0.92-7), and lmtest (version 0.9-35) packages. RESULTS: The isotemporal reallocation showed that for manipulative skills, an increasing pattern was observed (0.14, 0.28, and 0.42-units) when reallocating 5, 10 and 15 minutes, respectively, from light PA to SB. CONCLUSIONS: The current study highlights that school-time composition is a significant predictor of FMS. Moreover, a modest increase in SB, at the expense of LPA, during the school-time may elicit a positive change in manipulative skills.