Located 120km off the Yorkshire coast, the wind farm is expected to serve around 800,000 homes in the UK.
Hornsea is expected to be the largest offshore wind farm in the world, the company said.
Dong Energy expects to make a final investment decision on the project in the second half of 2016.
Dong Energy Wind Power senior vice president Mats Vikholm said: "Our decision to make Siemens the preferred supplier for our Hornsea Project One wind farm is a crucial step in our efforts to lower cost of electricity.
"The 7MW turbine is an upgrade of the 6 MW platform which we know very well from Westermost Rough in the UK and Gode Wind 1+2 in Germany.
Siemens will also supply turbines for the 580MW Race Bank project and the 330MW Walney Extension phase 2.
