The 50-kilogram weapon was found during construction work in Kölner Straße near the Frankfurt Messe area in the west of the city.
Police announced that an area within a 500-metre radius of the bomb would be evacuated. All residents are expected to have evacuated the area by 7pm. Bomb specialists will then move in to defuse the Fliegerbombe on Thursday evening.
All public transport in the affected area will be closed down on Thursday, including the S3, S4, S5 and S6 suburban lines and the Galluswarte station, local outlet the Frankfurter Rundschau reports.
Whoever needs assistance in their evacuation from the area can call the city authorities on 069 212-111.
There are still thousands of unexploded bombs under German cities, dropped by allied air forces during the Second World War.
In September the biggest ever post-war evacuation took place in Frankfurt when a 1.8-tonne bomb was found in the city. Some 60,000 people were evacuated on that occasion.