Le Minh Hai, deputy director of Lao Cai's Hydrometeorology Centre, said it was unusual for Vietnam for snow to fall at this time of year, which is classed as late spring or early summer.
"Snow falls normally occur from December to February," he said.
Hai said the snowflakes stirred excitement among locals and tourists.
The last time snow was recorded in Sapa, which is 2,000m above sea level, was in January 2003, but it was only a light fall.
Hai said a strong cold spell that had hit the north had brought the snow.
A Vietnam News Agency correspondent in Sa Pa said tourists were heading to the hills to watch the event, creating congestion on several roads.