The brief is to deliver social engagement that broadens the charity’s appeal and will focus on communicating the importance of "special places" in people's lives.
The charity already has a social community of just under 400,000 members. Jam will be tasked with increasing that number as well as monitoring key audiences.
Mark Scott, marketing director at the National Trust, said: "Our social channels have developed really well over the last couple of years and will be an increasing focus for us.
"Through its strategic and creative thinking, Jam will be key in helping us continue to evolve perceptions of the Trust and opening up conversations with new and existing audiences."
Jam’s first task is to develop the creative platform supporting a national brand campaign, which kicks off this summer.
At the end of last month, the charity appointed WPP agency Mindshare to handle its £3m consolidated media planning and buying business. The account was previously divided between Carat and M4C.
The National Trust’s ad account will continue to be handled by 18 Feet & Rising.
Richard Costa D’sa, managing director of Jam, said: "The Trust has a unique role in terms of the country’s relationships with special places.