Cystic hygroma (CH) is a benign infiltrative malformation of the lymphatic channels. We report a case of a 28-year-old Indian female who presented with rapidly enlarging right sided neck swelling over the posterior triangle since 5 days. Complete resection of CH is sometimes not amenable because of its infiltrative nature and involvement of surrounding vital structures. However, in our patient successful complete surgical resection was undertaken. The MRI findings of our patient were consistent with brachial cleft cyst; this posed a challenge in the diagnosis of CH. The histopathological analysis of the resected mass confirmed CH. CH is rare in adults and such an acute presentation is exceptionally atypical. History of prior trauma and infection are known etiological factors for adult CH; these were conspicuously absent in our patient. CH should be considered in the differentials of rapidly enlarging cystic swelling of posterior region of neck in adults. Optimal and timely management is necessary to achieve a favorable prognosis. Therefore, we report a case of rapidly enlarging cervical CH in an adult along with extensive literature review to have a better understanding regarding epidemiology, etiopathogenesis, clinical presentation, optimal management, and prognosis of such a rare entity in adults.