Birmingham Airport has become a one terminal facility, with managers claiming it will give passengers a better journey.
Terminals one and two have merged to form one terminal for all arrivals and departures.
All departing passengers will now use a new enlarged combined security search area with self-service and manned check-in desks.
Inbound passengers will arrive into a single meeting and greeting area.
Further work will see the opening of a new duty free shopping area and fashion house in the next few months.
Will Heynes, the airport's operations director, said: "The £13m investment puts the airport in a stronger position to attract new customers and showcase the airport as the region's global gateway.
"Our goal for 2011 is to improve the passenger experience and this project will create a smoother journey through the terminal and improve our overall efficiency.
"Over the next 10 years we will invest a further £100 million in our facilities, which will help us achieve our vision of doubling our passenger numbers by 2018."
He added: "Although the scheme will not be fully completed until July this year, the transition into a single terminal facility is a major milestone for the company."