The first of five jacket foundations was installed on 26 July. The 1,500-tonne steel structure will support Alstom's 6MW Haliade turbine. Block Island is located five kilometres off the Rhode Island coast, northeast US.
Installation of the foundations is expected to take eight weeks over the summer months. Turbine installation will follow in summer 2016.
The foundations have been built by Gulf Island Fabrication, at its facility in Louisiana, southern US, with some components being made by Rhode Island business Speciality Diving Services.
Bureau of Ocean Energy Management (BOEM) director Abigail Ross Hopper said the installation recognises Deepwater Wind's vision, determination and hard-won success for constructing the nation's first offshore wind farm. "It also represents a major step forward in standing up a sustainable offshore wind programme for the Atlantic Coast," she said.
Developer Deepwater Wind made a final investment decision on the $290 million project in March.
In a country with almost 67GW of onshore wind capacity installed, it has been a long road for the offshore industry in the US.
For a long time, it looked as if the 468MW Cape Wind project in nearby Massachusetts would be the US's first offshore development. However a string of court cases delayed the project so much it lost its power purchase agreements with local utilities, effectively placing it on life-support.
The path for Block Island has also had its difficulties. In August 2013, opposition from local residents forced Deepwater Wind to withdraw its proposal to run an electric transmission cable through the town of Narragansett in Rhode Island. The cable route was moved six kilometres south.
The project was also slated to use Siemens 6MW turbine, but the deal was never made firm before opting for Alstom's 6MW turbine in 2014.