Calls for universal health coverage (UHC) have triggered renewed focus on strengthening primary health care (PHC) and improving health worker performance. In Kenya, health service delivery is impeded by the poor quality of service, inefficient use of resources and low health worker motivation among other factors. Result-based financing (RBF) has the potential to strengthen service delivery by improving motivation, quality and resource use. The RBF concept typically entails deploying some form of financial reward towards staff who achieve a certain level of performance or outcomes. Studies on RBF in low and middle-income countries have given mixed results. This study looked at the effect of RBF on the motivation of PHC staff in Makueni County. Specifically, it sought to determine the effect of RBF, operational funds and service improvement funds on staff motivation. The study was conducted in the six sub-counties in Makueni County using a descriptive research design. Data was collected using a questionnaire, and descriptive and inferential statistical analyses were done. The study found a strong positive correlation between performance-based incentives and healthcare workers’ motivation. Also, the study revealed that performance-based payments and motivation of healthcare workers had a strong positive relationship. The study found that operational funds and motivation of healthcare workers have a strong positive relationship. The study established that service improvements funds had a strong positive correlation with the motivation of healthcare workers. The study recommended that health facility management consider performance-linked incentives for staff, and that effort is made to ensure availability of operations and service improvement funds for smooth running of the facilities. This would contribute to the UHC goal of ensuring access to safe and high-quality healthcare services.