Saints sign Sevens star
The 28-year-old last week featured for his country at the Rio 2016 Olympics, helping Argentina to a sixth-place in the sport's return to the Games.
He was capped by the Argentina XV side once in 2008 and was also part of his country's squad for the 2016 Americas Rugby Championship.
Estelles has also featured regularly for Argentina in the Sevens World Sevens Series over the past six months.
He joins from Atletico del Rosario, one of Argentina's oldest clubs, and is relishing the opportunity that has been given to him at the Saints.
"I would like to thank the Saints coaching staff for giving me this opportunity to join the club," said Estelles.
"I am delighted to have this fantastic chance to challenge myself and prove what I can do at the highest level in England.
"I am aware of the great number of world class players that the club has at its disposal and I know that there will be plenty of competition to earn a starting jersey. But I believe in my abilities and I will be working hard to make the most of this experience, on and off the field.
"I'm looking forward to meeting my new teammates and I will be doing all I can to help the club as we strive to achieve our goals."
Saints Director of Rugby Jim Mallinder says that Estelles has the potential to have an impact in the English Premiership.
"Juan Pablo was part of an impressive Argentina squad at the Olympics so we know that he has the fitness levels to go straight into the Premiership," said Mallinder.
"He's a powerful and elusive runner with a good set of handling skills as well as strong defence.
"We are looking forward to seeing what he can do here at Franklin's Gardens."
