Tunnicliffe began his career at Stockport County before joining the Seagulls in July 2009.
He made his Hatters debut at the age of 16 against Notts County in 2005 and had a loan spell at Norwich in 2007.
The Dons have a number of injury problems in defence, with Sean O'Hanlon, David McCracken, Mathias Doumbe and Darren Powell all sidelined.
And assistant manager Karl Robinson said he was delighted with Tunnicliffe's arrival at Stadium MK.
"James is a player we've admired for a long time. He's a big, strong lad but also a very good footballer," he told the club's website.
"The injuries we've suffered in defence this season have been unbelievable so it was essential to bring in a centre-half."
Tunnicliffe has made 17 League One appearances for Brighton this season, scoring two goals.
Source: BBC Sport
Source: BBC Sport