Dominion said the exact turbine model and volume will be decided at a later phase of the development, but if it selects SGRE's current 11MW direct-drive turbine, it would mean 240 machines could be installed at the site 43km off Virginia Beach.
SGRE is supplying two 6MW turbines to the Coastal Virginia Offshore Wind (CVOW) project currently under construction off Virginia.
CVOW will be the first offshore wind capacity to be added in the US since the 30MW Block Island site in 2016.
The developer said the deal will form part of the permit application for the 2.64GW project it intends to submit to the Bureau of Ocean Energy Management (BOEM) later in 2020.
The two firms will also look at supply chain development opportunities in the state, Dominion said.
"Siemens Gamesa... has proven to be a great partner in our CVOW project," said Mark D. Mitchell, Dominion Energy's vice president of generation construction.
Dominion filed an application with regional transmission company PJM in September 2019 to interconnect the three-phase project to the grid.
It plans to start ocean survey work next year and submit a construction and operations plan by 2022.
The development would comprise three 880MW phases for a combined capacity of 2,640MW, due online by 2026.
Virginia is targetting 30% of its power from renewables by 2030.