The 48-year-old, who was World Boxing Council champion for 10 months in 2006, was arrested in 2016 following allegations that he had abused the girl over two years, starting when she was just seven in 2012.
He was unanimously convicted of "repeated sexual abuse" of a minor.
When the trial began on Thursday, Baldomir showed his middle finger to those present, a provocative gesture recorded by television cameras.
Baldomir's greatest career highlight came in January 2006 when he shocked the unified welterweight champion Zab Judah.
After a successful defense of his WBC crown against the late Arturo Gatti, Baldomir lost his title to Floyd Mayweather in a November 2006 unification bout.
He retired in 2014 with a record of 49 wins, 15 by knock-out, 16 defeats and six draws.
He started his career as a trainer and in 2016 was given an award by the Argentine congress for achievement in sport.
Baldomir had been held in pre-trial detention since his arrest but before that was living with his wife and four-year-old son. He has three daughters from his first marriage.