The Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, have been pressing an operation to capture the terrorists’ stronghold since last year, and entered the city in June.
"The Al-Yarmuk district was liberated Sunday," the SDF's spokeswoman for the Raqa operation Jihan Sheikh Ahmed told AFP.
Al-Yarmuk is a large neighbourhood on the southwestern outskirts of the city.
"The operation is continuing but there are many fierce clashes," Ahmed said, speaking in the town of Ain Issa, some 50 kilometres (30 miles) north of Raqa.
"We are taking steady and sound steps. What is important to us is not speed, but liberating civilians and eliminating Daesh," she added.
The Britain-based Syrian Observatory for Human Rights said the SDF had advanced in Al-Yarmuk but did not yet fully control the district.
The monitor said the militia held the western portion of the district but that heavy fighting was continuing.
It also reported that hundreds of civilians had fled Daesh-held parts of the city towards areas now controlled by the SDF in the last 48 hours.
The monitor estimates the US-backed force currently holds around 35 percent of the city.
The SDF began an operation to capture Raqa in November 2016 and spent months taking territory around the city before finally entering it in June.