The new lanes, located between Sandown Road and Knock Road, operate on the Upper Newtownards Road as part of the Belfast Rapid Transit scheme.
They operate both ways during peak hours from 7.30 am to 9.30 am and from 3.30 pm to 6.30 pm Monday to Friday.
The Department for Regional Development said the bus lanes “will help to provide a more reliable service”.
A DRD spokesperson said: “Each new measure that supports our bus services helps to promote public transport and provide an attractive travel alternative for the public.
“These additional bus lanes will help to provide a more reliable service, which will benefit the increasing number of bus passengers choosing to use this route.”
The BRT will link the city centre with west and east Belfast and the Titanic Quarter.
The east Belfast route will link the city centre to the park-and-ride facility in Dundonald via Albertbridge Road and Upper Newtownards Road.