Aim: To analyse, in schoolchildren, the relationship between daily steps with metabolic parameters; and to examine whether this association is mediated by cardiorespiratory-fitness (CRF). Methods: A cross-sectional analysis of baseline data from a feasibility trial was performed in children from two primary schools in Cuenca, Spain. Daily steps were measured using the Xiaomi MI Band 3. Lipid and glycaemic profiles were analysed from blood samples. CRF was assessed using the 20-m shuttle run test. ANCOVA models were used to test the mean differences by daily steps quartiles. Mediation analyses were conducted to examine whether CRF mediates the association between daily steps and lipid and glycaemic parameters. Results: A total of 159 schoolchildren (aged 9–12 years, 53% female) were included in the analysis. Schoolchildren in the highest daily steps quartiles (>10 000 steps) showed significantly lower triglycerides and insulin levels (p = 0.004 and 0.002, respectively). This association did not remain after controlling for CRF. In mediation analyses, a significant indirect effect was observed through CRF in the relationship between daily steps with triglycerides and insulin. Conclusion: Children who daily accumulate more than 10 000 steps have better lipid and metabolic profile, and CRF mediated their relationship in schoolchildren. © 2023 The Authors. Acta Paediatrica published by John Wiley & Sons Ltd on behalf of Foundation Acta Paediatrica.