Police said most roads will remain open throughout the event but some closures and diversions are in place in parts of the city centre and east Belfast.
The race began at 09:00 BST at Ormeau Park and is expected to finish at the same location at noon.
Full details of the roads affected are on the Belfast City Marathon website.
The area around Ormeau Embankment will be closed to through traffic in both directions until noon.
"The area around Victoria Street will be disrupted between 8.30am and 11am," a police statement said.
"Disruption can also be expected on Grosvenor Road, Great Victoria Street, and Bruce Street heading towards Dublin Road, Shaftesbury Square and Botanic Avenue from 8.30am to 11.50pm."
The race also led to disruption in parts of east Belfast, including the Albertbridge Road, Newtownards Road, Dee Street and Sydenham Bypass, but this was expected to be cleared by about 10.15 BST.
Thousands of people are taking part in a half marathon in Belfast on Sunday and motorists have been advised to expect traffic disruption on the route.