The one-way system along North Esplanade and the Quay, in St Peter Port, will be reversed - to run northbound from the Town Church to the Weighbridge Roundabout - because of road works.
Quay Street and Town Church will also be re-opened.
The diversion will be in place from early morning Saturday 21 March, until Sunday 29 March.
It's because resurfacing works need to be completed on the seaward side of the junctions at either end of the works.
Inbound services to the Bus Terminus (apart from the 93) will follow the diversion, or join the diversion at various points.
Pedestrians should follow signs to the appropriate crossing points.