Rosario were crowned with the new title after Angel Di Maria's side finished the regular season of the Apertura and Clausura tournaments with a combined 66 points, four clear of Boca Juniors.
Under previous regulations, the team with the highest annual points tally earned only a Copa Libertadores berth.
Platense, who won the Apertura in June, will meet the Clausura champions — to be decided through playoffs starting on Saturday — in the end-of-year Champions Trophy.
"The Rosario fans must be very happy," said World Cup winner Di Maria, who claimed his first Argentine title.
Rosario added another trophy to their haul that includes seven domestic cups and one CONMEBOL Cup.
"We had a great tournament. I thank (AFA president) Claudio Tapia and all the leadership for their recognition," manager Ariel Holan told reporters.
AFA clarified that the new title would not mean an extra place was available for the continent-wide Copa Libertadores.
"The creation of the 2025 League Champion does not invalidate the realisation of the already established International Super Cup, Argentine Super Cup and Champions Trophy as well as the future realization of the Cup Winners' Cup," AFA said in a statement.
"It is also established that the 2025 League champion does not free up a quota at all for CONMEBOL tournaments."