The 2005–06 Argentine Primera B Nacional was the 20th 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.
This leg was played between the Apertura Winner: Godoy Cruz; and the Clausura Winner: Nueva Chicago. The winning team was declared champion and was automatically promoted to 2006–07 Primera División and the losing team played the Second Promotion Playoff.
This leg was played by Nueva Chicago, the losing team of the Promotion Playoff, and Belgrano, who was the best team in the overall standings under the champions. The winning team was promoted to 2006–07 Primera División and the losing team played the Promotion Playoff Primera División-Primera B Nacional.
Nueva Chicago won 6–4 on aggregate, becoming the 2nd. team promoted to Primera División
It was played by the teams placed 3rd, 4th 5th and 6th in the Overall Standings: Chacarita Juniors (3rd), Huracán (4th), San Martín (SJ) (5th) and Talleres (C) (6th). The winning team played the Promotion Playoff Primera División-Primera B Nacional.
The Second Promotion playoff loser (Belgrano) and the Torneo Reducido Winner (Huracán) played against the 18th and the 17th placed of the Relegation Table of 2005–06 Primera División.
Note: Clubs with indirect affiliation with AFA are relegated to the Torneo Argentino A, while clubs directly affiliated face relegation to Primera B Metropolitana. Clubs with direct affiliation are all from Greater Buenos Aires, with the exception of Newell's, Rosario Central, Central Córdoba and Argentino de Rosario, all from Rosario, and Unión and Colón from Santa Fe.