Erskine cares for around 1,400 veterans of all ages in homes throughout Scotland, providing long-term respite and dementia care in new purpose-built homes in Bishopton, Erskine, Glasgow and Edinburgh.
It is understood to have worked with the incumbent Carat for 12 years before appointing MediaCom.
David Shearer, managing director of MediaCom Edinburgh, said: "We’re really pleased that we will be able to bring our extensive charities experience to bear on Erskine’s media planning and buying to help them better support more ex-servicemen and women in need."
Sarah McNaught, marketing manager of Erskine, said that this year the charity needs to raise over £8m in voluntary donations to care for over 1,400 ex-Service men and women in its five care homes across Scotland.
She said: "We are delighted to be working with MediaCom and, with their sector experience, are confident that they will enable us to generate great results."