In this paper, the authors present a case diagnosed as ‘metaplastic thymic sarcoma’ presenting as a thyroid mass: thymic-like tumours of the thyroid are extremely rare. The reported case concerns a seventy-eight-year-old woman, who presented with a rapidly enlarging neck mass. Histological diagnosis was made challenging by the unusual biphasic characteristics of the tumour; the diagnosis of ‘metaplastic thymic sarcoma’ was eventually established, following consultation with an internationally recognised soft tissue pathologist. Review of the relevant literature demonstrated no comparable cases, and it is believed to be a unique entity. The patient suffered aggressive recurrence of the disease shortly after her surgery. Next-generation sequencing indicated a BRAF mutation, and there was a dramatic clinical response to Dabrafenib/Trametinib treatment. The successful use of such therapies is an encouraging finding.