The Withania somnifera, also called Ashwagandha, is available everywhere in the world. We present a rare case of thyrotoxicosis following Ashwagandha administration, specifically painless thyroiditis (PT) in this report. The patient was a 47-year-old previously healthy Japanese man, who started taking Ashwagandha two months before his first visit to our hospital. He visited our hospital for typical thyrotoxicosis symptoms like a sense of fatigue, fever at night, and weight loss followed by diarrhea and headache. Blood tests disclosed thyrotoxicosis. Thyroid ultrasonography showed internal echo heterogeneity and no increase in blood flow. Thyroid scintigraphy revealed a deficiency in thyroid uptake. Based on these findings, he was diagnosed as PT. After stopping the administration of Ashwagandha, both his symptoms and serum thyroid markers were improved. This report may spark important debate about whether ashwagandha is safe among healthy people, especially in thyroid toxicity.