Scientific reasoning is defined as the intentional pursuit of knowledge and the articulation of theory and evidence. Mastery of this skill entails the formulation of hypotheses, the design and execution of experiments, the evaluation of results, and the formulation of conclusions. These domains can be grouped into different components, as outlined in the Science-K Inventory (SKI): experimentation, data interpretation, and understanding the nature of science. The present study aimed to (a) analyze the psychometric properties and gather validity evidence for the Brazilian Portuguese version of the Science-K Inventory (SKI), and (b) investigate scientific reasoning in children aged 7–10 years and its association with intelligence, reading fluency and advanced theory of mind. Children from two public schools in the São Paulo metropolitan region participated (N = 145; 43% girls), aged 7–10 years (mean = 8.8; SD = 1.1). The instrument demonstrated adequate reliability indices (α = .79; ω = .82; split-half = .78). Correlations with reading fluency (r = .39; p < .001), intelligence (r = .43; p < .001), and theory of mind (r = .21; p < .01) were observed. Additionally, age (F = 11; p < .001) and grade level (F = 14.4; p < .001) had effects on the scores. A linear regression model demonstrated that reading fluency and intelligence, but not theory of mind, explained 23% of the variance in scientific reasoning (R² = .23). The Brazilian SKI represents the first validated instrument to assess scientific reasoning in the Brazilian children. Replicating established findings with an underrepresented population supports the parsimony of the scientific reasoning theoretical model. The availability of reliable measures of predictive learning skills may inform early intervention initiatives, strengthen evidence-based educational practices, and improve the science and technology curricula.