The LSU was one of four agencies that pitched for the account. The pitch was handled by Creativebrief.
The appointment does not affect Rainey Kelly Campbell Roalfe/Y&R's position as the retained creative agency for the charity after it won the account last year.
Last August it produced a print campaign, 'Every Man Remembered', that remembered First World War soldiers by using celebrity namesakes. Last week that campaign was nominated for a Campaign Big Award.
LSU will now conduct a strategic overview of the organisation, which supports serving members of the armed forces, veterans and their families, with a view to rethinking the brand and putting together an advertising strategy by October.
The agency will also work to develop the Legion’s case for support as it approaches the centenary of the end of the First World War.
Gary Ryan, The Royal British Legion's director of marketing and communications, said: "LSU quickly understood the core of the challenges facing the Legion.
"Their agile approach is exactly what we need in what is now a highly competitive third sector. We’re really excited to have access to their strategic expertise and creative thinking."
For the year ending September 2014 the Legion spent just under £10m on communications and campaigns.
Matt Boffey, LSU’s managing director, said: "The Royal British Legion is an inspirational organisation that does so much good work that people don’t hear enough about."