The Morgulduin was a river of Gondor that began in Morgul Vale in the Ephel Dúath. It ran westward past Minas Morgul until it reached the Cross-roads in Ithilien, where its course turned southwest until it entered the Anduin just south of Osgiliath. Just to the north of the stream was the road from Osgiliath to Minas Morgul.
It is probable that the original name of the river was Ithilduin and that it was renamed Morgulduin at the same time as Minas Ithil was renamed Minas Morgul after its capture by the Nazgûl in T.A. 2002.
On March 9, T.A. 3019 as Frodo, Sam, and Gollum were approaching the Cross-roads they heard the voice of the river, which sounded cold and cruel. At the time of the War of the Ring the water from the river was not suitable for drinking. Near the bridge over the river some way into the Morgul Vale cold vapours issued from the river who sickened Frodo on March 10.