The 26-year-old back-row forward will leave Newcastle Falcons in the summer to join the ambitious Headingley club on a two-year deal.
The signing of a man who played more than 100 times for Saracens, winning the Premiership title in 2011, is a significant one by Carnegie, who have ambitions to make the top two in the Championship next season.
The news offsets the disappointment of losing Paul Hill, the 20-year-old who has a world junior championship winners’ medal and a bright future, and whose move to Premierhsip champions Northampton Saints this summer has been confirmed.
“I’d like to thank everyone at Yorkshire for the help and support they’ve given me over the years,” said Hill, who joined Carnegie as a 15-year-old after learning the game with his home-town club Doncaster Juniors.
“But I’m looking forward to challenging myself at one of the top clubs in Europe and learning from experienced international players.”
The incoming Saull said: “I have spoken to (head coach) Bryan Redpath and (director of rugby) Chris Gibson and they set out their vision for the future and their plans to get in to the Premiership. It is a very exciting time to be joining the club.”
Carnegie host play-off bound London Scottish tomorrow, while the now-safe Doncaster Knights can climb to ninth by beating today’s opponents, Bedford.
Doncaster chief Clive Griffiths said: “Retaining our place in the Championship following promotion is a fantastic achievement and to finish just in or outside the top eight would be really good.”