Dozens of soldiers and IS fighters were reportedly killed.
Sirte is IS's most significant stronghold in Libya. It has been under IS control since February 2015.
Libyan government forces announced on their official Facebook page that they also had control of an official radio station and power management facility.
Al-Jazeera reports that the troops have also recaptured an IS ammunition store.
Troops loyal to the unity government, which was formed in the capital Tripoli earlier this year, had already re-captured the port and other parts of Sirte.
Meanwhile, a mystery explosion in a town close to Tripoli has killed at least 25 people, apparently after a dispute at a shop escalated.
The blast took place in Garabulli after clashes between locals and militia fighters from the city of Misrata.
Officials initially said a munitions store had exploded but one resident told the BBC a lorry loaded with fireworks had exploded.(BBC)