The South African born right-hander, who is due to tour Zimbabwe with the England one-day party later this year, has played the last four seasons at Trent Bridge.
He was signed by former team manager Clive Rice and because of his English mother he was able to play on a British passport.
Notts' Director of Cricket Mick Newell said he was extremely disappointed to lose a man of Pietersen's ability.
"We offered him a new three-year deal on a considerable salary, which would have maintained his status as one of the highest players at the club, but he has chosen not to accept it," he said.
"It's very disappointing because from a pure cricket point of view, I can't see what else we could have done to make him stay.
"Our pitches and practice facilities have been rated among the best in the country this season and the team has been highly successful.
"We've also signed a highly-rated Test skipper in Stephen Fleming in addition to Graeme Swann and most cricket observers rate Notts as one of the most improved teams in the country."
Newell added that Pietersen clearly had personal reasons for leaving and had expressed a desire to move to the south of the country.
"Unfortunately, the geography of our ground is one thing we have no control over," said Newell.
"Kevin appeared happy last season and played his part in a very successful team on the field and in a buoyant dressing room.
"While we are very disappointed to loose him we wish him well in his future England career and hope that his time at Notts has played a significant part in his progression to the national one-day squad.
THURSDAY 21 SEPTEMBER 2017
ENGLAND v WEST INDIES