The Â£68m scheme between Leigh and Manchester is expected to cut journey times by up to 30 minutes.
Eight buses per hour will serve the four-and-a-half mile (7km) stretch along a disused railway line.
The scheme has three park-and-ride facilities for more than 400 cars and links to bus priority routes through Salford on East Lancashire Road.
The new section is part of a Â£122m 14-mile (21km) bus priority route connecting Atherton, Leigh and Tyldesley with Salford and Manchester city centre.
Work began on the guided busway in July 2014 but was criticised by local businesses and residents following significant delays.
Andrew Fender, chairman of the Transport for Greater Manchester committee, said: "By prioritising buses, we not only make journey times quicker, more reliable and more attractive for passengers, we also help to tackle congestion and reduce emissions.
"I am grateful to those communities for their patience while the scheme was being completed."
A new guided busway aimed at reducing congestion in Greater Manchester has opened.