A 42-year-old multi para, presented with a mass protruding from vagina since 1 year. The mass was cystic in nature arising from the posterior vaginal wall. It was insidious in growth, gradually increasing in size to 8×8 cm. Surgical excision of the cyst was done by sharp and blunt dissection. The cyst was filled with mucoid material and histopathology confirmed a mullerian cyst. Cystic lesions of vagina are relatively uncommon and they are an incidental finding during gynaecological examination. Mullerian cysts are congenital cysts of vagina, usually reported during childbearing age group. These cysts usually arise at the level of cervix and extend anteriorly in relation to bladder, but very rarely they may also extend posteriorly. This paper illustrates a rare case of large posterior vaginal wall cyst of Mullerian origin.