Curcuma longa L. (Zingiberaceae), known as turmeric, is a perennial tuberous plant from the genus Curcuma, which includes about 100 plant species. The chemical composition of the turmeric rhizome is very diverse. Diarylheptanoid derivatives, also known as curcuminoids (of which curcumin, demethoxycurcumin and bisdemethoxycurcumin are the most important representatives), are the major active constituents of the plant rhizome. Many extracts used in the food and pharmaceutical industries are produced from thermally processed rhizome, when there are significant changes in the composition of the main compounds. Therefore, the aim of the study was to compare how the type of thermal treatment affects the content of curcuminoids and the antioxidant properties of the rhizome. The plant material was subjected to three different methods of thermal processing—microwave heating, boiling and frying in different time intervals. The chemical composition and antioxidant activity of the processed rhizome was evaluated using LC-MS (liquid chromatography–mass spectrometry), HPLC (high-performance liquid chromatography) and spectrophotometric methods (a DPPH test and TPC assay). Obtained results revealed that curcumin was the major curcuminoid present in all samples (113.92 mg/g of the fresh rhizome). Significant correlation between the type and time of the thermal processing and the composition of turmeric samples was revealed. A traditional boiling process lasting for 10 min was the most beneficial process in terms of the curcuminoid content (204 mg/g of curcumin) and antioxidant activity of the samples.