<b><i>Objective:</i></b> We aimed to evaluate whether radiotherapy causes changes in the mineral composition, hardness, and morphology of enamel and dentin of primary teeth. <b><i>Materials and Methods:</i></b> Thirty specimens of primary teeth were subjected to radiotherapy. At baseline and after 1,080, 2,160, and 3,060 cGy, the specimens were subjected to microhardness, FT-Raman spectroscopy, and scanning electron microscopy (SEM) analysis. The pH of artificial saliva was determined, as were the calcium and phosphate concentrations. The data were subjected to the Shapiro-Wilk normality test, showed a nonnormal distribution, and were compared by the Kruskal-Wallis test. <b><i>Results:</i></b> The results showed that the microhardness of the enamel surface decreased after 2,160 cGy (281.5 ± 58 kgf/mm²) when compared to baseline (323.6 ± 59.5 kgf/mm²) (<i>p</i> = 0.045). For dentin, the surface hardness decreased after 1,080 cGy (34.9 ± 11.4 kgf/mm²) and 2,160 cGy (26 ± 3.5 kgf/mm²) when compared to baseline (56.5 ± 7.7 kgf/mm²) (<i>p &#x3c;</i> 0.0001). The mineral and organic contents of phosphate (<i>p &#x3c;</i> 0.0001), carbonate (<i>p &#x3c;</i> 0.0001), amide (<i>p</i> = 0.0002), and hydrocarbons (<i>p</i> = 0.0031) of enamel decreased after 3,060 cGy (5,178 ± 1,082, 3,868 ± 524, 999 ± 180, and 959 ± 168 kgf/mm², respectively). For dentin, we noticed a growing increase in phosphate <i>v2</i>, amide, and hydrocarbon content after 1,080 cGy (8,210 ± 2,599, 5,730 ± 1,818, and 6,118 ± 1,807 kgf/mm², respectively) and 2,160 cGy (1,0071 ± 2,547, 7,746 ± 1,916, and 8,280 ± 2,079 kgf/mm², respectively) and a reduction after 3,060 cGy (6,782 ± 2,175, 3,558 ± 1,884, and 3,565 ± 1,867 kgf/mm², respectively) (<i>p &#x3c;</i> 0.0001). SEM images showed cracks on enamel and degradation of peritubular dentin. <b><i>Conclusion:</i></b> We concluded that radiotherapy caused a reduction in surface hardness, changed mineral and organic composition, and promoted morphological changes on the enamel and dentin of primary teeth.