The 21-year-old, who has appeared 82 times for Brighton, played five games on loan at Karl Robinson's Championship strugglers earlier this campaign.
The England Under-21 international is eligible for Saturday's FA Cup third-round match against Northampton Town.
"I've been here before, I know what the club is all about and I just can't wait to get started," he said.
Robinson added to the club website: "I felt he was a real success in his first spell with us earlier this season and this was highlighted when we looked back and saw he was the top of our attacking stats for a midfield player.
"When he was here before, Darren Potter wasn't fit and Samir Carruthers wasn't playing in central midfield and, as a group, this time around I'm sure they will offer good balance."
Find all the latest football transfers on our dedicated page.
MK Dons have re-signed Brighton & Hove Albion midfielder Jake Forster-Caskey on loan for the rest of the season.