The US National Hurricane Center says the Category 4 storm made landfall around 7 a.m. EDT (11am GMT) Tuesday near Les Anglais, Haiti.
Matthew's maximum sustained winds are near 145 mph (230 kph).
In the capital of Port-au-Prince, residents walked about in flooded streets.
The hurricane has pounded the south-western coast of Haiti, threatening a largely rural corner of the impoverished country with devastating storm conditions as it heads north toward Cuba and the eastern coast of Florida.
Click here for the latest on the track of Hurricane Matthew.