Desmoid fibromatosis is a very rare neoplasm. Intrathoracic desmoid tumour is even rarer subset of this group and only a handful of cases has been reported in medical literature till now. We here report the case of a 34 year old female from Eastern India who presented with refractory pain in the left axilla and inner arm. Thoracic imaging revealed a large mass in left lung apex surrounding the great vessels and brachial plexus nerves. The mass was completely excised surgically and histopathology revealed spindle-shaped cells in a collagenous background. Beta-catenin was strongly positive and SMA was focal positive. The patient was not given any radiotherapy. Intrathoracic desmoid tumour is extremely rare but timely treatment can be rewarding. Immuno histo chemistry (IHC) plays a significant role in its diagnosis. Clinicians should be aware of this rare neoplasm because it usually requires urgent surgery. The patient should be followed up for local recurrence.