Its new ranking emerged in a Civil Aviation Authority report on airport and passenger activity for 2016.
In the league table of 47 UK airports, Belfast International comes in behind Heathrow, Gatwick, Edinburgh and Glasgow in terms of passenger flow.
Overall, the airport grew by more than 17% last year with over 5.1 million passengers through the facility.
That figure is expected to grow further in 2017, despite the decision by United Airlines to end its trans-Atlantic service months after a Â£9m rescue deal.
Managing Director Graham Keddie said it was "an outstanding result" for the airport.
"Passengers are voting with their wallets. They want value for money and frequent services at the best times of the day," said Mr Keddie.
"Small to medium businesses are doing more business in GB and we're able to get them to their target markets at competitive rates with easyJet and Ryanair.
Mr Keddie said the airport could do "much more with the right supports".
"Air passenger duty is a considerable disadvantage, not only to our airports, but to Northern Ireland as a region," he said.
"If it disappeared, we could transform inbound tourism, leading to further significant investment along with the creation of thousands of jobs. Getting rid of this impediment is, in my view, a no-brainer."
Speaking about the end of the United Airlines service, he said: "We're working closely with government to deliver a major long-haul project that would open up attractive additional and badly-needed connections."
Belfast International is now the fifth busiest airport in the United Kingdom for domestic passenger traffic.