The fruits of the common quince (Cydonia oblonga) have wide-ranging health benefits due to their valuable composition. However, consumers usually do not welcome the hard flesh and astringency, so the fruit is not eaten raw. Therefore, it is important to choose the proper method for processing, including heat treatment, to preserve the high quality of the common quince fruit. The study examined the effects of freeze drying and convection drying at two temperatures (40 °C and 60 °C) on selected physicochemical, bioactive, and antioxidant properties of Cydonia oblonga fruits. It was found that freeze drying allowed the processed fruit to retain properties most similar to fresh fruit. This variant had the highest rehydration rate (3.53 ± 0.04), the lowest shrinkage rate (9.87 ± 0.29%) and the lowest bulk density (0.41 ± 0.01 g/cm3). Freeze drying preserved the brightest fruit colour (L* = 75.70 ± 1.71). These samples also had the highest total acidity (1.34 ± 0.01 g/100 g DM). Drying reduced the fruit’s tannin content, but no statistically significant differences were detected between freeze-dried and convection-dried samples at 40 °C and 60 °C. Freeze-dried quince fruits retained a high content of polyphenols (233.56 ± 5.96 mg GEA/100 g DM), flavonoids (36.79 ± 0.74 mg EPI/100 g DM), and antioxidant activity against ABTS (364.51 ± 9.12 µM Trolox/100 g DM) and DPPH (258.78 ± 5.16 µM Trolox/100 g DM). The highest losses of the mentioned bioactive compounds, and thus of antioxidant activity, were recorded in samples dried by convection at 60 °C.