The main purpose of this study was to investigate the impact of pupil-teacher ratio (PTR) on academic performance in mastering reading, writing, and arithmetic competencies in public primary schools in Morogoro Municipality. The study targeted 62 public primary schools in the municipality, in which a total of 5 schools were sampled for the study. A descriptive survey design was used as the research design for the study. Interviews and documentary reviews were used in collecting data for the study. Interviews were conducted with the municipal primary education officer in charge, all head teachers, and all standard four teachers. Documentary reviews were done on Standard Four National Assessment and analysis reports and documents concerning the number of teachers and pupils in sampled schools. Piloting study of the research instruments was conducted prior to the actual data collection to determine the validity and reliability of the tools. The data collected were analysed descriptively. The finding of this study revealed that PTR significantly influences the performance of pupils in standard four-nation assessments and in mastering reading, writing, and arithmetic competencies. The study recommended that all education sector stakeholders pay adequate attention to PTR since it affects the performance of pupils in primary schools. The study recommended the government employ more teachers to lower PTR and ease teacher workload. Also, the government through the MoEST and PO-RALG should undertake a balancing of employed teachers throughout the country to ensure equity in teacher distribution thus bringing down PTR in those schools which have high PTR and reducing teachers in those schools that have been overstaffed. Furthermore, the government should review education acts to empower head teachers to admit pupils on the basis of PTR in the respective schools