The 24-year-old has signed a five-year deal at the City Ground after scoring 17 goals and adding 13 assists in 56 appearances for the Argentine top-flight club.
The forward, who can also play as a winger, has won 14 caps for Paraguay and impressed in the recent Copa America.
“Ramon has been coveted by many clubs but was so clear in his desire to join Nottingham Forest,” said Ross Wilson, Forest’s chief football officer.
“We could all feel his energy, emotion, pride and excitement to be here from the first minute he arrived.
“He can’t wait to get started and we are delighted that he is here with us.”
Sosa began his career in his native country where he won the Copa Paraguay with Olimpia in 2021 before moving to Argentina.
An impressive return of six goals and seven assists in his debut season with Gimnasia led to a move to Talleres.
Sosa arrives on the eve of Forest’s opening Premier League game, at home against Bournemouth.
The signing follows Forest’s recent deals to bring in Sosa’s fellow winger Jota Silva, midfielder Elliot Anderson, defenders Nikola Milenkovic and Eric da Silva Moreira, and goalkeeper Carlos Miguel.
