Argentina striker Martin Palermo ended a run of eight matches without a goal by netting twice for Boca.
Newell's Old Boys, Banfield's only remaining challengers who began the day two points behind, also lost 2-0 at home to San Lorenzo, for whom Fabian Bordagaray struck twice.
Newell's would have had to win their match and Banfield lose for the team from Rosario to win their sixth crown.
At the Boca's Bombonera ground, Palermo converted a sixth-minute penalty to the joy of Newell's fans in Rosario 350-km north of the capital.
Newell's goalkeeper Sebastian Peratta then saved an early penalty from former Argentina winger Kily Gonzalez only for Bordagaray to put San Lorenzo in front in the 10th minute with a shot in the top corner and raise a cheer from Banfield fans down in Buenos Aires.
Palermo grabbed his second for Boca on the hour from defender Gabriel Paletta's downward header following left back Claudio Morel's cross.
Bordagaray scored his second midway through the second half from defender Pablo Pintos's cross and missed out on a hat-trick when his shot in the 75th minute hit the angle of post and bar.
Banfield's Uruguayan striker Santiago "Tank" Silva was the 19-match tournament's top scorer with 14 goals.
Colon midfielder German Rivarola was suspended for four matches after kicking the ball at referee Javier Collado and striking him with it after the final whistle in their 1-0 home defeat by Newell's in Santa Fe last month, the AFA said.
