A combined study of gamma radiation and radon measurements was conducted in eight post-Byzantine churches in Thessaloniki. The average total absorbed dose rate due to gamma radiation was measured at 32 ± 7 nGy h−1. The contributions of various radionuclides to the total absorbed dose rate from gamma radiation were estimated as follows: 40% from 40K, 32% from the thorium series, and 28% from the uranium series. These contributions align well with previous measurements in buildings in Thessaloniki. Gamma spectrometry measurements performed at the center and edges of the churches to identify potential deviations due to changes in geometry. It was revealed that total dose rate at the edges was 20% higher than at the center of the churches. Radon detectors were placed in the main nave and in the women’s galleries. The average radon concentration was 81 Bq/m3, with a geometric mean of 70 Bq/m3 and a range of 26–148 Bq/m3. Radon concentration did not exceed the upper limit of 300 Bq/m3 in any church. Across all churches, the radon concentration in the women’s galleries was 30% higher than in the main nave. A weak correlation was observed between the absorbed gamma dose rate due to the uranium series and the radon concentration. These findings enhance our understanding of natural radioactivity in historic buildings and provide a baseline for future radiological assessments in cultural heritage sites.