The 25-year-old, who has recently signed a short-term deal at Championship promotion hopefuls Bristol, will move to Franklin’s Gardens this summer.
Kessell had been impressing for Cornish Pirates before agreeing to join Bristol until the end of the season.
And Saints boss Jim Mallinder believes the player has the ability to succeed in England’s top flight.
“Tom has shown great promise in the Championship and has had his performances recognised with selection for the team that played Canada last year,” Mallinder said.
“We believe that he has the potential to perform in the Aviva Premiership and in Europe and are looking forward to seeing him develop here at Franklin’s Gardens.”
Kessell, who has previously represented England Counties, is excited by the prospect of moving to the east midlands.
He will face a fight against the likes of Kahn Fotuali’i and Lee Dickson for the Saints No.9 shirt.
And he said: “Joining the Saints is a fantastic opportunity for me. I’m really looking forward to working with the calibre of players that the club’s already got.
“Hopefully I’ll be able to develop as a work in progress, take any opportunity that I can get, and be the best that I can.”