Warwick Brady, the chief executive of Stobart Group, which owns and operates London Southend Airport, said Ryanair's move would help the airport's strategy to welcome more than five million passengers a year by 2022.
The news follows warnings the airline planned a pivot away from the United Kingdom thanks to Brexit, and news that just 10% of Ryanair's growth will be in the United Kingdom this year, compared to 25% in Germany.
Ryanair will close its Glasgow base at the end of the summer, and demand for flights to the United Kingdom and Europe is reportedly flat this year.
The airline's chief commercial officer, David O'Brien, said: "Don't confuse this great news as some kind of vote of confidence in the future of United Kingdom aviation".
Ryanair will begin operating flights from London Southend Airport next year, it has announced.
Ryanair's latest expansion will see three aircraft based at the regional airport, delivering an estimated one million passengers on 13 new routes including Alicante, Barcelona Reus, Bilbao, Brest, Corfu, Faro, Milan Bergamo and Dublin.
He said Ryanair was only expecting to grow its United Kingdom business by 4% compared with 21% in Germany in 2018.
O'Brien said disruption from European air traffic control strikes, primarily in France, could be the "worst ever" for Ryanair, which cancelled 1,100 flights in May.