MK Dons have signed defender Joe Walsh from Crawley on a three-year contract for an undisclosed fee.
The 23-year-old, who played twice for the Dons in a loan spell between March and April, played over 100 matches for Crawley over three years with the club.
"I'm delighted to get this deal sorted," Walsh told the club website. "I enjoyed my time here on loan.
"It supported what I already knew - this is a club going places and I'm looking forward to being a part of it."
He added: "I'm really excited about playing in the Championship and I'm already itching to get back for pre-season.
"We've got a talented bunch of lads here and I'm sure we won't just be making up the numbers."
Manager Karl Robinson, who guided MK Dons to the second tier of English football for the first time, said that taking Walsh on loan had been with a view to sign him permanently.
"Walsh gives us something we haven't had for a while in terms of being a left-footed centre-half," said Robinson.
"He impressed us both in training and on the pitch last season and he adds to a strong group of centre-backs that we know can compete for starting places."