Upper extremity adventitial cystic disease is rare, but the characteristic findings of this lesion should be known to the hand surgeon and used to guide treatment. We present a case of a young adult male who developed a painless mass in his distal forearm. Diagnostic imaging workup revealed a cystic mass that extended within and encased the radial artery. Both MRI and direct intraoperative visualization confirmed the presence of a stalk connecting the intra-mural radial artery mass to the radiocarpal joint. The mass and stalk were excised en bloc with fenestration of the volar capsule to prevent recurrence. This case demonstrates a less common example of upper extremity adventitial cystic disease and supports the articular theory of origin of these lesions. When surgical excision is performed, an attempt should be made to identify and excise the articular stalk in an effort to minimize risk of recurrence.