Background <p>All districts in Tanzania Mainland are endemic for soil-transmitted helminth (STH) infections, and have implemented at least five rounds of preventive chemotherapy targeting school-age children (SAC) in primary schools. However, there is limited information on the prevalence of STH infections in many parts of the country. Therefore, the present study was carried out to determine the prevalence and factors associated with STH infections among 5 to 14-year-old in primary schools in Machame Narumu, Hai District, north-eastern Tanzania.</p> Methods <p>A school-based cross-sectional study was conducted between May and June 2022. A two-stage random sampling method was used where one ward out of ten wards and four out of five primary schools were selected at stage one by simple random sampling technique During stage two sampling, the pupils from each of the four selected schools were stratified by their classes, then randomly selected based on the probability-proportional-to-size (PPS) method. A structured questionnaire was used to collect information on socio-demographic information and risk factors for STH infections from each participant. Stool samples were examined under the microscope following formol-ether concentration for the detection of helminth eggs. Data were analyzed using STATA Corp Software version 16.0. Logistic regression was employed to assess factors associated with STH infections among school-age children in primary schools, Hai District. Prevalence of STH infections was obtained by dividing the number of children who were infected with STH by the total number of children who participated in the study. A p-value &lt; 0.05 was considered statistically significant.</p> Results <p>The overall prevalence of STH infections was 26.6% (95% CI: 22.3%-31.4%), relatively higher than the national prevalence of 8.4% that was reported about 9 years ago. <i>Trichuris trichiura</i> was the most prevalent, with the prevalence of 10.2% (95% CI: 7.3%-13.7%), followed by <i>Ascaris lumbricoides</i>, 8.9% (95% CI: 6.2%-12.2%), and hookworms, 0.5% (95% CI: 0.1%-1.9%). However, multiple infections were also observed where children were infected by more than one roundworm species. Factors that were significantly associated with STH infections among SAC were age(AOR = 1.21, 95% CI: 1.03–1.42, <i>p</i> = 0.019), sex (AOR = 0.52, 95% CI: 0.29–0.92, <i>p</i> = 0.038), use of spring water (AOR = 0.41, 95% CI: 0.19–0.86, <i>p</i> = 0.86), and information about STH infections (AOR = 2.14, 95% CI: 1.01–4.53, <i>p</i> = 0.046).</p> Conclusion <p>Despite annual mass drug administration, STH infections remain a public health concern among school children in the study area, with moderate prevalence relative to the World Health Organization (WHO) thresholds of 20% to 50% prevalence. This calls for an integrated approach to include other interventions such as health and hygiene education, and programs for improving sanitation in the study area. This is the first report of STH infections in the Kilimanjaro region.</p> Clinical trial number <p>Not applicable.</p>