Baggage screening areas will also get an upgrade while the airport will buy a new fleet of airport fire vehicles.
The improvements will also see a 30%increase in retail space while food and drink areas will grow by 25%.
The amount of customer seating will also double, as will the number of airside customer toilets.
The completed works will be contained within the current terminal building and will be completed by October 2018.
Belfast City airport chief executive Brian Ambrose said: “In keeping with our overall mission of delivering an airport experience that exceeds our customers’ expectations, Belfast City Airport is fully committed to continually improving the overall journey for our passengers.
"The £15m investment in our infrastructure is another major reinforcement of our commitment.
“The upgrading of our departure lounge and retail offering will provide greater choice and significantly enhance the experience for our core business and leisure passengers as they travel through the airport.
“The strategic designs help future-proof the airport and we look forward to continuing what has been a successful period across many areas of our business.”