Sri Lankan Airlines will become a Oneworld member on May 1.
The carrier, which became a member elect of the alliance in mid-2012 (see news, June 2012), will offer Oneworld's complete range of services and benefits from that date.
The move means that the 100,000 members of Sri Lankan's Flysmiles loyalty programme will, in effect, have their frequent flyer privileges extended to whenever they fly with any Oneworld member airline.
Flysmiles Platinum cardholders will have Emerald status in the Oneworld programme, while Flysmiles Gold will be the equivalent to Oneworld Sapphire, and Flysmiles Classic will be Oneworld Ruby.
Kapila Chandrasena, Sri Lankan Airlines' chief executive, said: "As an airline highly focused on quality service, we are extremely proud to be lining up to become part of the highest quality alliance with the best airline partners offering a global network that best complements our own.
"We look forward to offering all the many benefits of Oneworld to our customers very soon now."
From May 1, Flysmiles Platinum and Gold members will be able to use any of the more than 600 airport lounges worldwide offered by Oneworld member airlines whenever they fly with one of the alliance's carriers.
Sri Lankan Business Class passengers will also be able to use Oneworld partner airline lounges.
And, the 150 million members of other Oneworld airlines' frequent flyer programmes will be able to earn and redeem awards and tier status points and receive all other Oneworld benefits on Sri Lankan.
Sri Lankan Airlines has seen a significant growth since peace returned to Sri Lanka four years ago.
It now operates a fleet of 21 aircraft from its Colombo base and flies to 34 destinations in 22 countries across Asia, Europe and the Middle East, including the Oneworld hubs of London Heathrow, Hong Kong, Kuala Lumpur and Tokyo Narita.
Sri Lankan's entry into Oneworld will bring three new destinations to the network - Kochi, Tiruchirappalli and Thiruvananthapuram, all of which are in Southern India and will expand the alliance's global coverage to 860 destinations in more than 150 countries.
TAM Airlines will join Oneworld on March 31 (see news, October 2013).