The National Centre for Hydro-Meteorological Forecasting said a strong western low-pressure system was driving severe heat across the regions.
In Hanoi and parts of the northern region, temperatures are forecast at 37-39 degrees Celcius, with some places exceeding 39 degrees Celcius.
From Thanh Hoa to the former Phu Yen Province in central Vietnam, temperatures are expected to range between 37 and 39 degrees Celcius, with some areas forecast to surpass 40 degrees Celcius.
Meteorologists warned that actual outdoor temperatures could be 2-4 degrees Celcius higher than official readings, especially in densely populated urban areas.
The heatwave in northern Vietnam is expected to last until May 27, while central regions could continue experiencing extreme heat until around May 28.
Authorities also warned that high temperatures combined with low humidity could increase the risk of fires, forest fires and heatstroke.
Meanwhile, the Central Highlands and southern Vietnam are forecast to experience scattered showers and thunderstorms in the evenings, with heavier rain expected from May 27 onward. Strong winds, lightning and hail are also possible during storms.