Saints have signed Jon Fisher following the back row forward’s successful trial period at the club.
The former London Irish player was involved in pre-season games against Rotherham Titans, Leinster and Newport-Gwent Dragons.
Fisher, who can play on the flank or at No.8, will help make up for the loss of Sam Dickinson, who is out for three months after having neck surgery.
And Saints boss Jim Mallinder says his new recruit deserved a permanent deal after displaying plenty of desire in training and friendly matches.
“Jon is a talented player who will give us depth across the back row,” said Mallinder. “He has worked hard throughout our pre-season training, putting in plenty of effort in both conditioning and rugby sessions.
“More importantly he has shown that he has the determination to play and succeed here at Franklin’s Gardens, and we were delighted to offer him this contract.”
Fisher, like Dickinson, has played for the England Saxons in the past and was once hailed as a future London Irish captain.
The 25-year-old is the second new signing at Saints this summer following the arrival of Joel Hodgson, and he is looking forward to helping the club build on last season’s success.
“I’ve had a great time at the Saints this summer, working with a superb group of coaches and outstanding players,” Fisher said.
“I had no hesitation in accepting Jim’s offer to join the squad full-time and am looking forward to helping the Saints maintain their place at the top of English rugby.”
Meanwhile, Saints have announced that Rob Verbakel has left the club.
The Kiwi lock, signed from Otago last summer, departed Franklin’s Gardens without playing any part in the pre-season preparations.