The government made the announcement after reassessing the "magnitude and severity" of the three-year drought. It has badly affected three of the country's nine provinces.
The decision came as Cape Town announced its water saving measures, which require each citizen to use less than 50 litres a day, had successfully pushed back "Day Zero" to 4 June. Just a matter of weeks ago, the date that Cape Town's taps were predicted to run dry was 12 April.
However, the decision to declare a national disaster means the central government - which is run by the African National Congress (ANC) - will now take responsibility for relief efforts.
