United have agreed terms on an £18.1million deal to sell Blind to Ajax, the club have confirmed.
Blind, 28, becomes Ajax's record signing in the move, which sees him return to the club he left four years ago when signed by then-United boss Louis van Gaal.
Blind was left out of United's squad for their pre-season tour of the US, signalling negotiations were coming to a climax as they flew out on Sunday.
And on Monday it was revealed Blind had already joined up with his new club, entering training with them ahead of the new Eredivisie season.
Blind signed for United in 2014 after a breakthrough World Cup with the Netherlands, where he finished third under Van Gaal, who then brought the player with him to Old Trafford in a £13.8m deal.