The Jaguares stunned the Lions 49-35 to claim their second win of the Super Rugby season in Buenos Aires on Saturday.
Javier Ortega Desio and Emiliano Boffelli crossed for two tries to see the Jaguares to a surprise win at the Jose Amalfitani Stadium.
The Lions recorded a 20-point win when the teams met a month ago, but there was to be no repeat.
The hosts crossed three times in each half, answering every time they were challenged by the Lions.
Malcolm Marx scored two second-half tries for the Lions, who twice fought back to within four points.
But the Jaguares pulled away again late, with Nicolas Sanchez finishing with 19 points.
The win sees the Jaguares sitting fourth in the South African Conference, which the Lions top.