The agency pitched against seven other shops, competing with Ogilvy Advertising in the final shoot-out. The contract will run for three years.
The agency's first campaign will aim to raise awareness of the charity, reinvigorate its image and encourage long-term involvement with Save the Children with a view to increasing donations.
Although no media strategy has yet been determined, online advertising is expected to play a major part in the campaign.
The contract with W&K is Save the Children's first following several ad hoc projects with a number of different agencies last year. They include Publicis, which created a child poverty campaign, and ongoing projects with the below-the-line agencies Proximity and Outrider.
W&K made the pitchlist after a recommendation from the branding agency Johnson Banks, which works with the charity on its brand identity.
The W&K managing director, Neil Christie, led the pitch process, alongside the creative director, Tony Davidson, and the planning director, Stuart Smith.
Joe Barrell, Save the Children's head of communications, said: "In the past, there has been cynicism towards change, but the public is beginning to realise it is within its power to demand and achieve it. Passive media isn't engaging for this purpose, so we're looking to use innovative channels. W&K has a track record of creating challenging relationships with its audiences as well as strong creative credentials.
"We need to communicate we're an independent children's movement away from any outdated perceptions of Save the Children as conservative or old-fashioned."
Christie added: "Save the Children has ambitious goals and we look forward to helping it ensure that those goals are accomplished."