Buenos Aires Mayor Mauricio Macri says there are suspicions that Wednesday's blaze was intentionally set.
Authorities say at least eight fire crews fought to bring the blaze under control after flames raged more than 30 feet (10 meters) in the air.
No injuries have been reported.
The Ecological Reserve covers more than a square mile (300 hectares) bordering the Rio de la Plata. The mayor says it hasn't yet been determined how much of the park burned.
The reserve is considered the "lung" of Buenos Aires, and its vegetation is home to lizards, snakes, turtles, frogs, bats, voles and weasels.
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