Background: The utilization of health research evidence is essential for informed decision-making, especially in health planning and achieving Universal Health Coverage (UHC). However, limited information exists regarding its use in health planning in Lower Middle-Income Countries (LMICs), including Tanzania. This study assessed the current use of health research evidence and factors influencing its utilization among health planning teams. Methods: A descriptive qualitative approach was used to examine determinants of evidence use in health planning. Data were collected through six focus group discussions and 34 in-depth interviews with planning team members from regional, council, and health facility levels. The COM-B model (Capability, Opportunity, Motivation) was applied to assess factors influencing evidence use, and thematic analysis was conducted on the transcribed data. Findings: While participants reported using available planning guidelines, the use of health research evidence in planning was low. Capability barriers included inadequate knowledge and skills for evidence-based planning. Opportunities included planning guidelines supporting evidence use, research coordinators, policymaker-researcher interactions, and infrastructure like computers and the Internet; however, access to knowledge translation tools was limited. Motivation was influenced by incentives such as extra duty allowances, promotions, and professional development opportunities. Conclusion: The use of health research evidence in planning processes in Tanzania is low, influenced by capability, opportunity, and motivation factors. Enhancing capacity-building, fostering collaboration, strengthening policy support, and improving infrastructure are recommended. Future research should evaluate the effectiveness of these interventions and their impact on healthcare planning and delivery. Efforts were made to minimize social desirability and selection biases.