The 26-year-old centre, who has also played for Leinster and Connacht in recent years, was an ever-present in Calvisano's Heineken Cup campaign last season, starting all six Pool Three matches.
Downey has penned a one-year contract with the Saints and is excited by the prospect of playing in England.
â00Northamptonâ00s a club with a strong tradition and fantastic, top-class facilities,â0 said the 6ft 4in Dubliner.
"Iâ00m looking forward to helping get the Saints back into the Premiership and meeting the challenge of playing against teams that will all be looking to defeat us. It will also be good to play with some familiar faces such as Christian Short and Damien Browne, who I played with at Connacht.
â00Playing for Jim Mallinder, Dorian West and Paul Grayson will be exciting for me. They will add to my game and improve it and Iâ00m relishing learning everything I can from them."
Saints' director of rugby Jim Mallinder added: "James brings plenty of experience and a good attitude to Franklinâ00s Gardens. He fits the profile of the player we want here, both in his hard work but also his ability to fit into an exciting back line and he could make a big impact this season. Iâ00m looking forward to welcoming him to the Saints.â0