The breathtaking structure is known as the Gate of God, and used to guard the ancient Assyrian city Nineveh.
The destruction of the ancient structure, also called the Mashki Gate, has been confirmed by the British Institute for the Study of Iraq, and the Antiquities Department in Baghdad has not denied the demolition.
The terrorists demolished the ancient gate using military equipment, according to activists in Mosul.
ISIS thugs have destroyed many of Iraqi historic sites and monuments, including the Assyrian city of Nimrud, the Winged Bulls, and the Mosul National Museum.
Other reports say the regime was planning to sell the separate blocks from the structure.
The fundamentalists have control over an estimated 4,500 archaeological sites and have pillaged and sold millions-worth of antiquities.
The historic Mashki gate, which was discovered in 1968, is believed to be one of the eastern Nineveh province.
“ISIS views tombs they destroy as sacrilegious and a return to paganism,” Syrian antiquities chief Abdul Maamoun Abdulkarim said.
Nineveh, mentioned in the Bible, dates to the 7th century BC, and was once the largest city in the world.
The destruction of the gate is just the latest in the series of acts of vandalism conducted by ISIS.
Syrian forces, with the aid of Russian military, seized control of the ancient town of historic Palmyra last month.
What they discovered when they entered the city were monuments destroyed or harmed, thousands of bombs and booby traps, ready to level the whole city.
They also found mass graves with dozens of tortured women and children, some only 500 yards from the ruins of ancient monuments.
Last year, ISIS extremists bombed the historic Yazidi ancient minaret of the Shingal district in northern Iraq, and a year ago, they blew up the church of Virgin Mary in the Assyrian village of Tel Nasri in north eastern Syria.