Anaplastic thyroid carcinoma (ATC) is the most aggressive type of thyroid malignancies and resistant to chemotherapy. Novel therapeutic strategy is required for better management of ATC. In this work, we show that artesunate, an antimalarial drug, is active against chemoresistant ATC cells. Artesunate dose-dependently inhibits growth and induces apoptosis in chemo-sensitive (8505C and KAT-4) and -resistant (8505C-r and KAT-4-r) ATC cells, and acts synergistically with doxorubicin. Using multiple xenograft mouse models, artesunate is active against chemo-sensitive and -resistant ATC cells in vivo at doses that do not cause toxicity in mice. Our mechanism analysis reveals that artesunate acts on ATC cells through suppressing mitochondrial functions without affecting glycolysis, leading to oxidative stress and damage, regardless of whether they are sensitive or resistant to chemotherapy. Interestingly, KAT-4-r cells demonstrate decreased glycolysis, increased mitochondrial membrane potential and mitochondrial respiration compared to KAT-4 cells whereas such phenomenon is not observed between 8505C and 8505C-r cells. This suggests that some but not all ATC cells gain enhanced mitochondrial biogenesis after prolonged exposure to chemotherapy drug, which may explain the different sensitivities of 8505C-r and KAT-4-r to artesunate. Our work demonstrates that artesunate is a potential addition to the treatment armamentarium for ATC, particularly those with chemoresistance. Our findings also highlight the therapeutic value of targeting mitochondria in chemoresistant ATC.