Reda Issa told CNN on Tuesday that forces corralled al-Jeeza al-Bahriya district of Sirte and they are now working to clear the area of mines and improvised explosive devices.
Eventually, he said, the "total liberation of Sirte" will be announced.
"People are shouting 'God is great,' " Issa said.
The former hometown of late Libyan leader Moammar Gadhafi, Sirte was the only major city outside Iraq and Syria under ISIS control. During this period, rights groups such as Human Rights Watch issued reports on atrocities committed by ISIS in Sirte.
In May, Libyan forces mostly from the city of Misrata but also from other western Libyan cities launched an operation to oust ISIS from the city. The operation was dubbed al-Bunyan al-Marsoos.
These forces, nominally aligned with the UN-backed government in Tripoli, faced fierce resistance. They were frequently attacked by suicide car bombs and snipers.
In August, the United States launched airstrikes that allowed Libyan forces to advance in their battle to retake the city.
According to Issa, at least 715 Libyan forces were killed and more than 3,000 injured since the military operations began.
While this might be the end of ISIS' control of major territory in Libya, it certainly is not the end of the group. ISIS still has a presence in different parts of Libya and the deteriorating living conditions, services and security that led to its rise has not changed.
Militia turf wars and political power struggles are common and over the past week intense clashes rocked the capital Tripoli. By some accounts the fighting was the worst in two years.