Netherlands international signs four-year deal with Amsterdam club.
Ajax have re-signed Daley Blind from Manchester United for a fee which could rise to £18.1m.
The 28-year-old returns to his boyhood club on a four-year deal after four seasons with United.
Blind confirmed the transfer in a statement on Tuesday, which read: "First of all I want to say how proud I am to be a part of the Manchester United family.
"To play for the biggest club in the world with such a history meant a lot to me. I had four great years with four trophies."
"I want to thank everyone, the managers, coaches, the medical department, everyone who helped me to settle in so quick at the beginning, of course my teammates and last but definitely not least the fans."
"It goes without question that everyone at the club wishes him well for the future," it read.