Underi (also called Jaidurg) is a fortified island near the mouth of Mumbai harbour south of Prong's Lighthouse.

It is a companion fort to Khanderi and currently lies in Raigad district, Maharashtra.

These islands of Khanderi and Underi served as one of the landmarks for ships entering Mumbai harbour.

[1] Underi is smaller than Khanderi.

The fortification was built by Kahim of the Siddis[2] in 1680 CE.