Oneworld airlines carry more than 500 million passengers a year on 3,300 aircraft with annual revenues of 140 billion US dollars.
SriLankan was officially inducted to oenworld at a ceremony in Mattala airport, the island's second international airport.
Senior minister for international monetary co-operation, Sarath Amunugama said the joining Oneworld will help Sri Lanka move on path to become an aviation hub.
SriLankan Airlines chairman Nishantha Wiclremasinghe said the alliance will connect the airline with 1,000 airports in 150 countries.
SriLankan flight UL318 to Kuala Lumpur at 7.45 am May 01 will be the first flight under the oneworld flag.
Bruce Ashby, chief executive of Oneworld said SriLankan was the first airline in the South Asian region to join the alliance.
The alliance now had no gaps in key regions in the world.
The alliance has members from North and South America, Europe, East Asia, China and Australia.