Dons boss Paul Tisdale admitted he has been eager to sign Baily Cargill for several years.
The 23-year-old defender became Tisdale's ninth signing since arriving at MK Dons when he put pen to paper on a deal - subject to EFL and FA clearance - on Wednesday.
Earlier this week, Dons confirmed the capture of Mathieu Baudry - another centre-back, while former Exeter City captain Jordan Moore-Taylor was the first signing through the door as Tisdale looks to shore things up at the back.
Tisdale said of his newest defender: "Baily is a player I've admired for a number of years, ever since I saw him play for England Under-20s, and the opportunity arose this summer to bring him here.
"He's a left-sided defender who has had lots of experience from loan spells but, I'm sure, will be pleased to have found somewhere where he can cement a place and demonstrate his ability over a period of time.
"I'm looking forward to seeing how he does, as he looks to take his career forward - I believe we're the perfect fit."
Tisdale added: "As I said after the signing of Mathieu Baudry, I wanted to build around our strength and, for me, this signing completes our versatility and depth in our defensive areas."