The agencies have already been briefed to work on a number of campaigns, working closely with Cancer Research UK’s in-house PR and social media team.
Nicola Dodd, head of PR and social media at Cancer Research UK, said: "Social media plays a crucial and proven role in our communications to supporters.
"In the past 12 months we’ve transformed the way we deliver social media, creating a structure that allows us to use engaging and inspiring content more effectively across all our platforms, have more news-driven conversations with our supporters and ensure that we are working seamlessly with our digital teams.
"Our new social media agency roster gives us additional support and creativity when we need it, especially for our biggest fundraising campaigns like Race for Life."
Last month the charity launched online platform Every Moment Counts, which showcases the experiences of cancer survivors.
Developed by advertising agency AMV BBDO, it contains a two-minute film called Moments, which can be shared through social media.