The Dutch boss has left Ajax and has been linked to Everton on several occasions.
The Dutch side lost out on the Eredivisie title on Sunday, denying De Boer his fifth title since taking charge in 2010.
He still had a year left on his contract, but will not be completing it, with his exit from the European giants being confirmed on Thursday evening by the club.
The news should be extremely interesting to Everton fans, with Roberto Martinez teetering on the brink at Goodison Park.
The Toffees were battered 3-0 by Sunderland on Wednesday in another embarrassing display from the side.
It leaves them mired in mid-table and finishing out of the top ten for the second successive season under the Catalan.
The clubs’ fans are now vociferously calling for his sacking, and De Boer is one of those that has been named as a potential replacement should that happen.
His representative, Guido Alders, fuelled that speculation by claiming his client would be interested in the Everton job to NOS, and his availability will only heighten that speculation.