Díaz made his debut for River Plate as a 17-year-old on Sept. 21, 2008.
The 22-year-old attacking midfielder will help bolster FCD's midfield, which has been short on depth this season. Diaz scored three goals in 53 appearances in the Argentine first division before being sent on loan to Chilean first division side Unión Española for the 2012 season.
“Mauro Diaz is a young prospect who fits the profile FC Dallas was looking to add,” said FC Dallas technical director Fernando Clavijo. “We’re excited to see how he will complement an already very talented offensive lineup as we head into the second half of the season with the goal of winning the MLS Cup.”
Diaz played 41 matches (26 starts) for Unión Española, scoring five goals and adding seven assists.