The Saints had hoped to keep the fixture at their Franklin's Gardens home, but the ground's 13,591 capacity falls below European Rugby Cup's 15,000 threshold for a last-eight tie.
The tournament organisers were unwilling to give special dispensation and, as expected, Northampton have now officially confirmed the move to the home of the MK Dons.
Chief executive Allan Robson said: "Although we are disappointed that we cannot hold the match in Northampton, we know that stadium:mk can be a great venue for us.
"It is a lovely stadium with outstanding facilities that is well run and the management there have told us that they want to make it a home from home for both the team and supporters for the day.
"The extra capacity available to us means that all of our season ticket holders, members and friends and family will be able to see the match, as well as many more supporters who would not otherwise have been able to go."