Installation of the first 6MW GE Haliade turbine was completed at 08.30 local time (13:30 UK time), according to Deepwater Wind CEO Jeff Grybowski.
The nacelles were shipped from GE's Saint-Nazaire facility last month and arrived in Rhode Island, northeast US on 1 August.
A total of five turbines will be installed 26km off the US mainland by Fred Olsen Windcarrier's Brave Tern jack-up vessel.
The jacket foundations were installed over the summer of 2015.
— Jeff Grybowski (@JGrybowski) August 4, 2016
The project will provide electricity to more than 1,000 inhabitants of the 25-square-kilometre Block Island, which currently ferries in a million gallons (4.5 million litres) a year of pricey, dirty diesel.
Offshore wind is beginning to gather pace in the US. The Bureau of Ocean Management has auctioned approximately nine zones for offshore wind development, with a tenth off the coast of New York expected before the end of 2016.
At the start of August, nearby Massachusetts passed a bill requiring local utilities to buy power from up to 1.6GW of offshore wind by July 2027.