Here, we take a look a back at the road that led these two clubs to the Wembley showpiece, which takes place on Sunday 26th February 2017.
Northampton Town 1-3 Manchester United
The Premier League giants avoided a potential banana skin by running out 3-1 winners at Northampton, who had upset higher-tier opposition in the two rounds prior.
Michael Carrick's goal was cancelled out, but Ander Herrera and Marcus Rashford struck in the second half to see United through.
Southampton 2-0 Crystal Palace
It was an all-Premier League tie for Southampton, their campaign getting under way with a routine home victory against Crystal Palace.
Charlie Austin's penalty got the ball rolling, before Academy graduate Jake Hesketh sealed it.Round Four
Manchester United 1-0 Manchester City
A derby in Round Four pitted the red half of Manchester against the competition's reigning champions, after the blue side of the city defeated Liverpool at Wembley in 2016.
But the title would not be retained, with Juan Mata's goal sealing bragging rights and a place in the Quarter-Finals.
Southampton 1-0 Sunderland
It took a moment of magic to settle this tie, and it came from Moroccan midfielder Sofiane Boufal.
Making his first start for the Saints, the tricky 23-year-old cut inside and curled a delicious effort into the far corner.Quarter-Finals
Manchester United 4-1 West Ham United
Zlatan Ibrahimovic started and finished the goalscoring on a comfortable night for the Red Devils.
The Swede scored in the second minute and the last, with Anthony Martial also bagging a brace in the 4-1 success.
Arsenal 0-2 Southampton
Southampton's first tie away from St Mary's produced a familiar result – victory and a clean sheet.
Jordy Clasie and Ryan Bertrand struck in the first half to send the Gunners packing, and the Saints into their first League Cup Semi-Final since 1987.Semi-Finals
Manchester United 2-0 Hull City
Hull City 2-1 Manchester United
(Manchester United win 3-2 on aggregate)The damage was just about done in the first leg, when Juan Mata and Marouane Fellaini gave United a two-goal advantage to take to Hull.
The Tigers made life difficult for Jose Mourinho's men in the return fixture, but Paul Pogba's second-half goal proved decisive.
Southampton 1-0 Liverpool
Liverpool 0-1 Southampton
(Southampton win 2-0 on aggregate)Nathan Redmond struck at St Mary's to give Southampton the early advantage, and if they could achieve a fifth successive clean sheet in the reverse leg, then they'd be through.
They duly did, protecting their lead at Anfield for 90 minutes, and Shane Long's breakaway goal ensured their place in the Wembley showpiece.