The 20-year-old joins the club for the rest of the 2018 MLS season, and the Timbers have a purchase option on the player attached to the loan deal.
Despite breaking into the top side in 2016, Conechny has made just 14 appearances in the top division for San Lorenzo since then.
The midfielder was a highly regarded prospect in Argentina and has made over 20 appearances for his country at various youth levels.
“We are pleased to announce the addition of Tomas Conechny to our group during this summer transfer window,” said Timbers general manager Gavin Wilkinson in a team release. “He is a player we have been following closely, and we believe he has a bright future ahead of him as a player who can play any of the attacking positions.
"We have been looking to add a young, talented dynamic player and feel that, in Tomas, we have a player who can help contribute short term, while also giving us a long-term solution in several positions. The opportunity to bring him to Portland was a unique one, and the fact he had already been watched live many times made this possible when we had to move quickly to acquire him.”
The Timbers already boast a number of Argentine players, including Diego Valeri, Sebastian Blanco and Lucas Melano, who is currently on loan at Estudiantes.
Portland currently sits fourth in the Western Conference, eight points back of conference leader FC Dallas with two games in hand.