The Department of Irrigation says minor flooding is possible within 48 hours in low-lying areas of the Kalu Ganga basin and Maguru Ganga basin.
The warning follows rainfall in upper catchment areas and the opening of spill gates at the Kukule Ganga Reservoir.
Areas including Bulathsinhala, Madurawala, and Palindanuwara DS divisions may be affected.
The Department has advised residents and motorists to take precautions, with Disaster Management authorities alerted.