A church and addiction treatment centre will be set up.
Another focus will be addressing violence against women and children.
The church was first approached in 1984, but finally decided to go, after being asked last year by Samoa's Prime Minister, Tuila'epa Sa'ilele Malielegaoi.
Tuila'epa cited ongoing problems with drug addictions and alcohol abuse.
The Salvation Army's head of social services, Colonel Melvin Fincham, said they were starting their operations with three church staff in Apia.
