Kydd was due to begin a two-year stint at Franklin's Gardens at the start of the campaign but a serious knee injury sustained while training with former club Saracens in March scuppered the deal.
The 23-year-old utility back, assisted by Saints' medical team, has since made a full recovery and has now signed a one-and-a-half-year deal.
Northampton's injury problems means Kydd, a former New Zealand under-21 international, will start Saturday's Guinness Premiership clash with Worcester at inside centre.
Saints director of rugby Budge Pountney said: "Robbie has done fantastically well to regain his fitness from what could have been a career-ending problem.
"He has been helped and guided by the physiotherapy department and the doctors, and they are more than happy to pass him fit.
"When Robbie said he would be fit by Christmas nobody really believed him but he almost managed it and now arrives at the time of an injury crisis."
Saints have added a new name to their casualty list following the news Paul Diggin has been ruled out for 12 weeks after damaging his medial ligament.