A spokesman for the Anbar province governor, Muhannad Haimour, confirmed the news: "Ramadi has fallen.
"Anbar operations command has been cleared. The city [Ramadi] was completely taken ... It was a gradual deterioration. The military is fleeing."
Ramadi is the capital of Iraq's largest province of Anbar.
The intense fighting coupled with suicide car attacks over the last two days in Ramadi have built up a death toll of an estimated 500 people that included militants and civilians, said Haimour.
The suicide car bombs continued on 17 May taking a toll on 15 Iraqi troops and police who were reported dead.
Meanwhile, the International Organization for Migration said over 8,000 people have been displaced in Ramadi in the last two days.
According to Haimour, the provincial command centre in the city is also under the Isis control with an officer captured inside reporting of mortars "raining down" with IS militants "in almost every street", reported Sky News.
Meanwhile, IS reportedly hailed the victory on a jihadist online forum with a message reading: "God has enabled the soldiers of the caliphate to cleanse all of Ramadi...after storming the 8th brigade. They (now) control it along with a battalion of tanks and missile launchers and in addition to the Anbar operations command."
Earlier on 15 May, the militant group raised their black flag over a local government building.
The reports are yet to be confirmed by Iraq's national government.