The fire brigade said two suspicious objects were found on the banks of the Elbe river near the Carola Bridge.
Investigators later confirmed one of them was an explosive device.
The 250kg British bomb is to be defused today.
Authorities said the evacuation would be the largest in Dresden since World War 2.
The evacuation zone covers a 1000m radius around the discovery site and includes major city landmarks such as the Frauenkirche church and the Semperoper opera house.
The police headquarters, the Saxon state parliament, several ministries, retirement and nursing homes, daycare centres and other social institutions are also located in the area.
Emergency accommodation will be available at the Dresden Messe exhibition centre.
The objects were discovered in an area where the Carola Bridge, which collapsed in 2024, is to be rebuilt.
Four bombs were found last August during demolition work on the bridge, when 17,000 people were evacuated.
Bombing of Dresden in British and United States air raids created a firestorm that destroyed about 90% of the city centre on February 13-15, 1945, killing up to 25,000 people. — TCA
