Background: Lenvatinib is an oral multitargeted tyrosine kinase inhibitor that has an anticancer action in patients with differentiated thyroid cancer that is refractory to radioiodine. Knowledge of the efficacy and safety of lenvatinib in patients with anaplastic thyroid cancer (ATC) is limited. Tyrosine kinase inhibitors frequently cause hypothyroidism, but the incidence of hypothyroidism with lenvatinib is unclear. Objectives: We conducted a retrospective study to investigate the efficacy and safety of lenvatinib in ATC. Methods: Five patients with unresectable ATC were enrolled. Lenvatinib 24 mg once daily was administered until disease progression, unmanageable toxicity, withdrawal, or death occurred. We retrospectively analyzed the objective response rate (ORR), time to progression (TTP), overall survival, and safety. Results: Three of the 5 patients (60%) had a partial response, and 5 (40%) had stable disease. The ORR was 60%. Median TTP was 88 days, and overall survival was 165 days. Hypothyroidism was a common treatment-related adverse effect; 4 patients (80%) had hypothyroidism of any grade. These 4 patients had not undergone total thyroidectomy prior to lenvatinib administration, and the other patient had undergone total thyroidectomy. Treatment-related adverse effects of any grade were hypertension in 80% of patients, diarrhea in 40%, fatigue in 80%, and decreased appetite in 80%. Conclusions: Lenvatinib is an effective treatment and may improve the prognosis of unresectable ATC. Four of the 5 patients had hypothyroidism, which may have been associated with treatment-induced injury of the thyroid gland. There were many treatment-related adverse effects, most of which were manageable by dose modification and medical therapy.