Heavy rain starting at around 9:15 p.m. in Bien Hoa Town in Dong Nai Province, a neighbor of Ho Chi Minh City, and lasting for over an hour submerged a section of National Highway 1 under half a meter of water for almost an hour.
Large vehicles were creating strong wakes, and one such caused the man's death.
Passers-by rushed him to hospital but in vain.
The flooding also stalled hundreds of motorbikes, causing severe traffic jams.
"It rained for almost an hour and the water drained too slowly," a man named Thang living in Bien Hoa said.
"The flooding caused many to fall off their motorbikes and people living along the road rushed out to save them."
For years the section of National Highway 1 in Bien Hoa has been suffering from flooding whenever it rains heavily.
The National Hydro-meteorological Center said the rainy season has begun in the south and localities there should expect rains in the afternoons and evenings.
The Vietnam Meteorological and Hydrological Administration had forecast rains would come earlier this year in the region.
Normally the rainy season begins in May.