Background <p>Group A rotavirus (RVA) is a leading cause of childhood diarrhea globally. Despite its status as a reportable infectious diarrhea in China, comprehensive and specific national RVA surveillance data remain scarce. This study analyzed the National Science and Technology Major Project (NSTMP) database (2009–2020) to characterize RVA epidemiology in Chinese children under 5 presenting with diarrhea.</p> Methods <p>RVA detection data from children under 5 with diarrhea were analyzed from the NSTMP database. Positivity rates and genotype spatiotemporal distribution were assessed using descriptive epidemiology. Temporal trends were evaluated with Joinpoint regression; significance of differences was assessed using chi-square tests.</p> Results <p>The overall positivity rate of RVA among children under 5 years old in China was 28.7% (15,945/55,637). The positivity rate in males (29.3%) was higher than that in females (27.6%). Positive cases were concentrated in children aged 6–23 months (70.4%, 11,231/15,945), with the highest positivity rate (35.5%, 6059/17,090) observed in the children aged 12–23 months. Joinpoint regression analysis revealed a sharp decline in the positivity rate of RVA from a baseline of 49.9% in 2009 to 23.1% in 2011, with an annual percent change (APC) of -31.9% (<i>p</i> = 0.04). Subsequently, between 2011 and 2020, the rate remained stable, showing no statistically significant change in trend (APC = -1.1%, <i>p</i> = 0.901). Additionally, the highest positivity rate of RVA was observed in winter (46.5%), while the lowest was recorded in summer (14.0%). Geographically, the highest positivity rate of RVA were observed in the central region (41.6%), while the eastern region exhibited the lowest positivity rate (20.0%) among the four regions in China. Furthermore, the distribution of genotypes exhibited geographical variation, with the proportions of the dominant G3, G9, and P[8] genotypes being lower in the eastern region than that in the central and western regions. Conversely, the P[4] genotype was more prevalent in the eastern region. As for the combination genotype, G9P[8] has increased markedly and become predominant circulating genotype.</p> Conclusion <p>RVA positivity rate in Chinese children under 5 declined significantly during 2009–2011 and subsequently stabilized, demonstrating distinct age, seasonal, and regional patterns with geographically variable dominant genotypes. Sustained surveillance is crucial to track evolving epidemiology and guide evidence-based vaccination strategies.</p> Clinical trial <p>Not applicable.</p>