After losing both Dylan Des Fountain and Roberto Quartaroli to season ending injuries, San Martin's arrival is a big boost to Rowland Phillips' side.
"I'm pretty excited to start this new adventure and I want to thank Aironi for this great opportunity," said San Martin.
"My target will be to reward this confidence and demonstrate to be able to play at this level, with an eye to World Cup.
"I know some of the Argentinan players in the Aironi roster so it will not be a problem to settle in the team and in Italy".
During last season San Martin played for Tala Rugby Club in Cordoba and joins an Aironi squad still searching for their first victory in Europe.
The Italian side went down 30 v 6 to Ulster on their Heineken Cup debut but a much improved performance against former European Champions Bath in their second outing will give them confidence ahead of their back-to-back fixtures against Biarritz Olympique later this month.