Anaplastic thyroid cancer (ATC) is the most lethal tumor arising from thyroid follicular epithelium. Lenvatinib is an off-label use option for ATC patients in many countries but an approved prescription in Japan. However, lenvatinib resistance is a substantial clinical challenge. Clinical ATC samples including lenvatinib-resistant tumors are used to build patient-derived cells and patient-derived xenografts. High-throughput drug screening and synergy analyses are performed to identify an effective combination partner for lenvatinib. Cellular functions are detected by cell senescence, apoptosis, cell cycle, cell viability and colony formation assays. CDK2 inhibition showed the significant synthetic lethality with lenvatinib via inhibiting G1/S transition and inducing cell senescence in ATC. High expression of CDK2 is associated with lenvatinib resistance and poor clinical outcomes of ATC patients. Lenvatinib increased protein expression of CDK2 in lenvatinib-resistant ATC cells. Mechanistically, lenvatinib inhibited protein degradation of CDK2 via reducing CDK2's interaction with the RACK1-FBW7 complex, which is involved in ubiquitination and subsequent proteasomal degradation of CDK2. Combination of CDK2 inhibitors in clinical trials (Dinaciclib or PF-07104091) and lenvatinib markedly suppressed growth of xenograft tumors from the lenvatinib-resistant patient. The findings support the combination therapy strategy of lenvatinib and CDK2 inhibitor for lenvatinib-resistant ATC patients with high CDK2 expression.