The 2003–04 National Division Three South was the fourth season (17th overall) of the fourth division (south) of the English domestic rugby union competition using the name National Division Three South. New teams to the division included Launceston who were relegated from the 2002–03 National Division Two while promoted clubs included Southend (champions) and Haywards Heath (playoffs) coming up from London Division 1 and Dings Crusaders as champions of South West Division 1. The league system was 2 points for a win and 1 point for a draw with the league champions going straight up into National Division Two and the runners up playing a playoff against the runners up from National Division Three North for the final promotion place.