The 27-year-old, who has won 22 caps for his country, has now turned down other offers from both France and England to make the switch this summer and join Warriors.
Creevy also captained Argentina when they played Warriors in a friendly at Sixways on the eve of the Rugby World Cup in 2011, where he featured in all five games for the Pumas.
Head Coach Richard Hill is delighted to agree terms for Creevy who joins countryman Leonardo Senatore plus experienced back Paul Warwick in agreeing moves next season.
"Agustin is a quality international player who will add real strength in depth and power to the pack of forwards we are building here at Warriors," said Hill.
"I really like the way he plays the game and the clear ball carrying ability he possesses. He will help us continue to develop our attacking game and has played some lovely stuff at Montpellier this season.
"Agustin is clearly very comfortable with the ball in hand and has very strong offloading skills. His ability in contact is excellent.
"He speaks very good English, which is important, and is an experienced international player who will know Leonardo (Senatore) so I am sure they play well together.
"We have signed a very passionate rugby player whose attitude and dedication is superb. I am sure he will be a big asset for the Warriors as we plan for the future."