The local weather office said the lowest temperature measured on Saturday was 10 degrees, and it warmed up to around 10.8 degrees at 7 a.m.
The town in Lao Cai Province stands 1,500-1,650 meters above sea level and is home to Fansipan, the highest mountain in Vietnam and Indochina peninsula.
Lao Cai Hydrometeorology Station said it was the coolest September in Sa Pa since 1957.
Tran Quang Nang, head of the weather forecasting department at the National Center for Hydro-Meteorological Forecasting, told VTCNews this was one of the early cold waves hitting northern Vietnam and more such waves are likely in the coming days.
National meteorologists have said that the cold wave could trigger rains, floods and landslides in Lao Cai and other localities in the region.
Sa Pa is one of a few places in Vietnam's northern highlands where it snows in winter, but this is a rare phenomenon.