Arup will lead on environmental impact assessments, looking at possible effects on the natural environment, biodiversity, water, geology, visual impact, towns, air quality and archaeology.
The contract is worth £7m.
"Arup will provide the technical leadership and oversight in the preparation of the environmental impact assessments," explained an HS2 representative.
The findings will inform a House of Commons bill, needed for official approval to start the line in 2017. Last month the coalition agreed in principle to the project and said it would draw up the bill.