The two sites - one in Ohio and one in Pennsylvania - have combined capacity of 100 megawatts (MW), enough to power 20,000 homes.
They were bought from renewable power developer Vesper Energy Development. Financial details of the transaction were not disclosed.
Octopus Energy is aiming to invest $2 billion in the U.S. renewables market by 2030. The company became Britain's largest electricity supplier this year.