The 22-year-old will link up with Karl Robinson again.
Brighton & Hove Albion midfielder Jake Forster-Caskey has joined Charlton Athletic on a permanent transfer, with the terms undisclosed.
The former England under-21 international returned to Albion from Rotherham United, after both clubs agreed to terminate his season-long loan agreement early to allow Forster-Caskey's move to the Valley, where he links up with Karl Robinson.
Forster-Caskey, who was limited to only six matches at Rotherham, had two successful loan spells at MK Dons under Robinson for a month in 2015, and again during the second half of last season.
Albion manager Chris Hughton commented, "There is no doubt Jake has a promising future in the game, but at this stage of his career he needs to be playing regular first-team football.
"On behalf of everyone at the club, I would like to wish him all the very best for the future, and thank him for his efforts while here at the club."
Forster-Caskey was Albion's youngest-ever player, when Gus Poyet gave him his first-team debut aged 16 against Yeovil Town at Withdean in 2010; which came a week after he had to get permission from his head teacher to travel with the first team to MK Dons.
He went onto make 64 starts, and 18 substitute appearances for the club, scoring nine goals in total; and also had a six-month loan spell with Oxford United in the first half of the 2012/13 season.
Forster-Caskey has also been capped 15 times by England at under-21 level, having represented his country at every age group since appearing for the under-16s.