The 2004–05 Argentine Primera B Nacional was the 19th season of second division professional of football in Argentina.
A total of 20 teams competed; the champion and runner-up were promoted to Argentine Primera División.
It was played by the teams placed 4th, 5th, 6th and 7th in the Overall Standings: CAI (4th), Atlético de Rafaela (5th), Nueva Chicago (6th) and San Martín (M) (7th).
The Second Promotion playoff loser (Huracán) and the Torneo Reducido Winner (Atlético de Rafaela) played against the 18th and the 17th placed of the Relegation Table of 2004–05 Primera División.
Since Chacarita Juniors and Defensores de Belgrano finished with the same relegation co-efficient at the dividing line, a one-match playoff was held to determine who had to play in the relegation/promotion playoffs and who had to be directly relegated to the Primera B Metropolitana.