The 28-year-old has been at his hometown club his entire career.
After coming into the Saints Academy system from Northampton club Boys Brigade Old Boys, Diggin made his first team debut in a Premiership Rugby match at Sale in February, 2004.
In the nine years since, Diggin has gone on to make 148 appearances in the Saints first team, scoring 62 tries.
His appearances include starts in the victorious 2010 LV= Cup, 2009 Amlin Challenge Cup and 2008 EDF Energy National Trophy finals, as well as the 2012 LV= Cup final and 2011 Heineken Cup final, which capped a year in which Diggin became the first Saint to score four tries in a single Heineken Cup match.
Northampton born and bred, Diggin is also about to take the first steps in a coaching career with the Saints Academy that he will combine with his playing commitments.
Saints chief executive Allan Robson says that Diggin deserves his testimonial year.
“Testimonials were created for players like Paul,” said Robson.
“He is a player who has come through the Academy to spend his entire career at his hometown club, and bleeds black, green and gold.
“Digger has always been wholeheartedly committed to helping the Saints succeed, and he has developed an outstanding relationship with the supporters throughout his decade in the first team.
“This testimonial year is well deserved, and I’m sure that all our fans will join me in wishing Paul every success with it.”
Events are likely to include a golf day, a day of shooting and country sports, a fun and activity day, special Twenty20 cricket and rugby matches, a gala black tie dinner and a ladies’ day, with full details to be announced in due course.