Fresh terms had been put to the 31-year-old midfielder in the wake of relegation out of the Premier League.
Cowie has considered all of his options and decided to call time on a three-year stint in South Wales.
He has been linked with a number of Cardiff’s Championship rivals as he prepares to take on a new challenge, along with a leading side in his native Scotland.
Blackburn Rovers and Watford are said to be keen on keeping Cowie in England, while Rangers would welcome the opportunity to take him north.
Cardiff boss Ole Gunnar Solskjaer is disappointed to be losing the services of an experienced head, but has drafted in Kagisho Dikgacoi this summer and is happy with the options available to him.
He said: “Don has been a great servant to the club and a fantastic lad to have in the dressing room.
“There was a contract there, but we have made signings now and Don wanted to keep his options open so all we can do now is wish him all the very best for the future.”