Sagar Island is an island in the Ganges delta, lying on the continental shelf of Bay of Bengal about 100 km south of Kolkata. This island forms the Sagar CD Block in Kakdwip subdivision of South 24 Parganas district in the Indian State of West Bengal. Although Sagar Island is a part of Sundarbans, it does not have any tiger habitation or mangrove forests or small river tributaries as is characteristic of the overall Sundarban delta. This island is a place of Hindu pilgrimage. Every year on the day of Makar Sankranti, hundreds of thousands of Hindus gather to take a holy dip at the confluence of river Ganges and Bay of Bengal and offer prayers (puja) in the Kapil Muni Temple. Kolkata Port Trust has a pilot station and a light house.

Gangasagar is a village and a gram panchayat in the Sagar CD block in the Kakdwip subdivision of the South 24 Parganas district in the Indian state of West Bengal.