The 2003–04 Argentine Primera B Nacional was the 18th season of second division professional football in Argentina.
A total of 20 teams competed; the champion and runner-up were promoted to Argentine Primera División.
Note: The series ended 2–2 on aggregate, and Almagro promoted to Primera after winning 4–3 on penalties.
The winning team played the Promotion Playoff Primera División-Primera B Nacional.
The Second Promotion playoff loser (Huracán (TA)) and the Torneo Reducido Winner (Argentinos Juniors) played against the 18th and the 17th placed of the Relegation Table of 2003–04 Primera División.