Uruguayan Lopez headed both goals in the final 16 minutes as Estudiantes retained a two-point lead over Velez Sarsfield, who also won 2-0 away to Racing Club in their final match.
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It was a nervous finish for Estudiantes with Velez taking a 23rd minute lead at Racing Club through striker Juan Manuel Martinez and increasing it to two goals when midfielder Maxi Moralez converted a free-kick in the 57th.
A win for Velez and a draw for Estudiantes would have taken the teams into a title play-off since goal difference is not taken into account.
Arsenal goalkeeper Cristian Campestrini had an excellent game before finally succumbing when Lopez, who had come on in the second half for injured captain Juan Sebastian Veron, headed home from a corner in the 74th minute.
Former Velez striker Lopez, who has seen little action this season due to injury, added another header two minutes from time then earned himself a yellow card for taking off his shirt in wild celebration.
"You have to give these players a standing ovation. We had lots of problems with injured and exhausted players but they managed to overcome them and the merit (of this title) is entirely theirs," coach Alex Sabella told reporters.
It is Sabella's second crown as Estudiantes coach after steering them to the Libertadores Cup, the South American title, last year.
It is the second time, including 2006, that Estudiantes have won the league title with Veron as captain. The club won their first title in 1967 with his father Juan Ramon Veron and Carlos Bilardo in the team.
Bilardo, who went on to become Argentina coach, steered them to two more titles in 1982 and 1983 with Sabella in midfield.
Velez, widely regarded as the best team of the championship, have Uruguayan striker Santiago Silva as the top scorer with 11 goals, one more than team mate Martinez.
Silva could be overtaken, however, with seven matches still to be played, two later on Sunday and five on Monday. Tigre's Denis Stracqualursi has nine goals.
