Background: Electronic health records and reimbursement claims data are increasingly recognized as valuable resources for enhancing health surveillance, particularly in evaluating continuity of care. However, their use is often complicated by coding errors. Objective: The aim of the study was to develop case definitions for a person living with HIV in Poland and for a newly diagnosed HIV case, based on claims data. Material and methods: Data on services and prescriptions reimbursed by the National Health Fund (NHF) between January 2010 and June 2022, in Poland, were used to construct candidate definitions for a person with diagnosed HIV (PHIV-D). Based on the selected definition and analysis of the time intervals between registered services, the size of the PHIV-D population residing in Poland was estimated. To evaluate the validity of the approach, the results were compared with national HIV surveillance data using mean squared error and visual trend analysis. Percentage growth was applied to assess the reporting delay in NHF data. Results: According to the selected definition a PHIV-D was defined as an individual who had used at least 2 services funded by NHF with an HIV-related ICD-10 code listed as either main or concurrent cause. PHIV-D who did not use any NHF-funded services or prescriptions for consecutive 24 months, and had no recorded death were considered likely to have left the country. Based on this definition, the estimated PHIV-D population residing in Poland increased from 8.9 thousand in 2013 to 15.7 thousand in 2021, with 10,993 new diagnoses recorded during this period. Conclusions: The developed definition allows to estimate the size of the PHIV-D population in Poland, which is challenging for existing surveillance system. It provides a valuable tool for monitoring and planning healthcare services for individuals living with HIV.