Background: Physical activity (PA) and sedentary behaviour (SB) assessment in children is critical for the prevention of noncommunicable diseases. African studies examining PA and SB of primary school-age children are few. This study investigated PA, SB levels and their correlates among primary school children in Lagos, Nigeria. Method: In a cross-sectional study of 733 learners, their self-reported PA and SB were assessed using the Children PA Questionnaire (CPAQ) (6-9 years age category) and Youth Activity Profile (YAP) (10-12 years age category) while pedometers were used for objective PA and SB assessment, and socioeconomic status (SES) index were measured using a structured questionnaire. Standardised procedures were used for anthropometric and cardiovascular measures. Results Based on CPAQ, 87.5% and 100% of the learners aged 6-9 years met the recommended PA and SB guidelines, respectively which were lower with pedometers (72.8% and 87.3%). The proportion of boys aged 6-9 years who met the guidelines for PA and SB (using pedometer) was significantly higher than that of the girls(PA: 80.7% vs. 64%, p = 0.018; SB: 94% vs. 80%, p = 0.008). Self-reported PA was positively associated with age (CPAQ: B = 455.39, p < 0.001; YAP: B = 1.638, p = 0.009) and negatively with SES (CPAQ: B = -201.39, p < 0.001; YAP: B = -1.000, p < 0.001). Objective PA was positively associated with waist to hip ratio(WHR) (6-9 years: B = 66090.24, p = 0.032) and negatively with sex (6-9 years: B = -5533.41, p = 0.027) and hip circumference (10-12 years: B = -1269.13, p = 0.017). SB was associated with SES in learners aged 10-12 years (B = -0.282, p = 0.003).Conclusion: High SES is a major predictor of reduced PA among these cohort of learners.