The authors report a rare case of actinomycosis, involving the thyroid gland in a 9-year-old female, presented with a hard, non-tender, non-inflammatory left neck mass of 4 weeks duration. On physical examination a firm, non-tender mass within the left thyroid gland was found; and the laboratory and imaging studies strongly suggested a thyroid neoplasm. After a left subtotal thyroidectomy, a gross sulfur granule was founded in the excised lesion. Histological examination confirmed actinomycotic thyroiditis. Actinomyces species are a very rare cause of infective thyroiditis, with difficulties in their diagnosis and treatment. The authors reviewed its pathogenesis and treatment.