The fly-half, who has been capped 56 times by Wales, will spend this season with Ospreys but is already looking forward to joining Saints.
The 27-year-old hopes to help Ospreys secure silverware in his final year with the club and has stressed he remains fully committed to Wales.
He said: "I'm hugely privileged to sign for a club with the history and tradition of Saints. It's extremely exciting to start a new chapter of my career with a club that shares my ambition to win trophies, which is where this club belongs.
"I have to say a massive thank you to the coaches and board for the confidence put in me and I look forward to working as hard as I ever have in my career to return the faith.
"This is an opportunity for me and my family to experience rugby in a different environment and playing in a town and surrounding area which has a massive passion for rugby was a huge attraction.
"I would like to thank everyone at the Ospreys and the Welsh Rugby Union who has supported and helped me throughout my career and an extra special thank you to the fans who have helped and supported me through highs and lows.
"To play for my home club was a dream come true, let alone to play as many times as I have done. I hope my passion for the shirt and people of this region has shone through in my performances and I would love to leave on a high note with some silverware. I also remain fully committed to Wales and achieving the best I can every time I pull on the red shirt."