The Magpies beat Reading in a thrilling penalty shoot-out on Friday night after extra time ended with the score at 2-2, with Callum Roberts and Kelland Watts on target for United.
Nathan Harker saved three spot-kicks in the ensuing shoot-out at the Madejski Stadium, with Jamie Sterry netting the winning penalty to earn Newcastle a shot at promotion.
And their reward for that win is a trip the south coast to face the Saints, who beat Aston Villa 2-0 in their semi-final, with goals from Will Smallbone and Tom O’Connor sending them through to the final.
The game will be played at St. Mary’s, due to Southampton finishing above Ben Dawson’s youngsters in the league.
Further details, including a confirmed date for the final, will be announced in due course.