On Monday morning torrential rains lashed the northern island in the Atlantic archipelago located off West Africa, swamping roads and sweeping away vehicles and people.
Municipal councillor Jose Carlos da Luz told a state broadcaster seven people had died in floods and one person was electrocuted, adding that three others were still missing.
In a report on Monday, the International Federation of Red Cross and Red Crescent Societies put the death toll at nine and said 1,500 people had been displaced on Sao Vicente.
Sao Vicente usually records 116 mm of rain in a year, according to Cape Verde's meteorology institute. But early on Monday 193 mm fell in just five hours, according to Ester Brito, an executive at the institute.
"It is a rare situation because what was recorded is above our 30-year climatological average," she told Reuters, adding that in just two hours more rain fell than the island typically receives annually.
Comment: A report from 4 days prior: 360,000 urged to evacuate in Kyushu, Japan as floods and landslides hit - record rainfall of 19.6 inches in 24 hours - twice the average for August