Now independent, the NCS is backed by a range of voluntary organisations and private sector partnerships.
It previously worked with Hill+Knowlton Strategies before holding a pitch for an agency to support its goals in 2014.
Forster has been tasked with reaching influencers, parents and teachers, as well as positioning NCS as a thought leader on youth issues.
NCS PR manager Carole Breen said: "In 2014, NCS aims to achieve the same substantial growth as we did in 2013 and in Forster we have an agency that understands our wider organisational goals and how to help us achieve them, beyond column inches alone."
Rosie Warin from Forster Communications said: "National Citizen Service is quickly becoming a well known national institution, with a vision to become a part of every teenager’s life experience.
"The organisation is growing fast and has ambitious targets to meet national demand. Our job is to make sure that parents, teachers and the wider public realise the benefits that the programme has on teenagers and, subsequently, on society."