The airline appointed the IPG Mediabrands agency following a four-way competitive pitch process.
UM London already provides media planning for the South African Tourist Board.
Jon Danks, the head of UK and Ireland marketing and communications at SAA, said this existing relationship made the agency more attractive.
Danks said: "A collaboration between a tourist board and an airline offers clear opportunities to understand the entire customer journey, and UM London’s ideas and overall approach was a clear winner.
"Digital and social marketing require an integrated campaign approach, agility, refinement and a clear retargeting strategy."
The agency will work alongside SAA’s existing agencies Black Diamond (PR), Spike (CRM) Street Designs (creative), and Beeliked (social).
Pete Skarparis, a planning manager at UM London, said: "Our existing expertise working with a client from this region in the holiday and travel sectors put us in a strong position. We very much look forward to a close working relationship."