According to the National Centre for Hydro-Meteorological Forecasting, the extreme heat was caused by the combined impact of a western low-pressure system and strong foehn winds.
Several weather stations in northern Vietnam recorded historic or near-historic highs during the heatwave.
In Quang Ninh Province, Uong Bi station recorded 38 degrees Celsius on May 25, surpassing the previous May record of 37.5 degrees Celsius set in 2013.
Hiep Hoa station in Bac Ninh Province registered 39.7 degrees Celsius, exceeding the previous record of 39 degrees Celsius recorded in 2019.
Bac Giang station recorded 39.4 degrees Celsius, breaking the former record of 39.1 degrees Celsius set in 2020, while Bac Ninh station matched its all-time May high of 40.5 degrees Celsius.
Hai Duong station in Haiphong also equalled its previous May record at 39.6 degrees Celsius.
Hanoi was among the areas hardest hit by the heatwave, with daytime temperatures frequently reaching 39-40 degrees Celsius and some areas approaching record highs.
Meteorologists said Hanoi faced more intense heat due to a combination of hot low-pressure systems, foehn winds and the urban heat island effect caused by dense concrete development.
Forecasters said extreme heat would persist across parts of northern and central Vietnam on May 27, with temperatures ranging from 38 to over 40 degrees Celsius.
However, the heatwave is expected to weaken from May 28 as cooler air moves southward from China, reducing temperatures in northern Vietnam to around 35-37 degrees Celsius.
Authorities forecast the widespread heatwave in northern Vietnam would end from May 29, while hot weather in central Vietnam is also expected to gradually ease.