The New Zealand scrum-half, who scored the Blues’ only try in their 11-5 semi-final win over Northampton, will join Bristol at the end of the season.
The former New Zealand A international is in his second season at the Cardiff Arms Park after three years at the Ospreys.
During his time at the Liberty Stadium, Spice lifted the Celtic League title in 2005 and 2007 and was part of the Blues team to reach successive Heineken Cup quarter-finals.
His departure follows the announcement that Wales scrum-half Gareth Cooper will join the region from Anglo-Welsh final opponents Gloucester next season.
His half back partner, Nicky Robinson is also on the move in the summer. He will make the reverse journey to Cooper, and will represent the Cherry and Whites next season.
