Mott MacDonald / Ramboll will provide structural, civil engineering, building services and airfield design work for the scheme, which is set to complete in 2022.
Mott MacDonald will also take on the role of project manager on the scheme and provide specialist input on constructability.
The work is part of a £600m transformation programme by Stansted owner Manchester Airports Group to increase capacity at the airport from 24m passengers a year to 35m.
Stansted started the bidding process for the expansion earlier this month.
The construction will be divided into two lots: one for the new arrivals lounge and one for the departures refurbishment.
The construction of the arrivals building will start later this year and is expected to take up to three years. The redevelopment of the existing terminal building is expected to complete in 2022.
Mott MacDonald aviation projects director Gary Howard said: “Our team has a shared vision and ethos for delivering the airport’s transformational programme, with proven experience of delivering projects in live airport environments.
“The redevelopment will support anticipated passenger growth and give users the high-quality facilities that they expect from an international airport.”