The Saints are trying to reach the final for the first time since 2003 and the winner of this match will face either Manchester City or Chelsea in the final on 15th May.
The Foxes are aiming to reach the final for a fifth time but are looking to win the competition for the first time. The club will want to end the unwanted record of reaching the most FA Cup finals without ever winning it.
The Saints have won the competition before, lifting the trophy back in 1976.
Both sides come into this one having very different Premier League seasons.
Leicester currently sit third in the table and are battling to finish in the UEFA Champions League places and will want to improve on the 3-2 loss away to West Ham United last weekend.
Southampton come into this one having a different season to the Foxes. Having had a brilliant start to the season even at one point sitting top of the table, there form has dropped off and now sit 11 places behind Leicester and come into this one after a really disappointing 3-0 loss at West Bromwich Albion.
James Maddison, Ayoze Perez and Hamza Choudhury are all available for selection this weekend after the trio were dropped for the game against West Ham after the trio breached Covid-19 regulations and attended a party.
Caglar Soyuncu is fit and available after returning a negative Covid test. The Turkish international contracted the virus whilst away with his national team.
His compatriot Cengiz Under is fit as well after recovering from injury.
Harvey Barnes is still a few weeks away as he recovers from injury but is recovering well according to Brendan Rodgers.
James Justin is the only other long-term injury.
Leicester XI: Schmeichel; Ricardo; Fofana, Soyuncu, Evans, Castagne; Ndidi, Tielemans, Maddison; Iheanacho, Vardy.
The Saints will be without on loan Liverpool midfielder Takumi Minamino for the game at Wembley as he is cup tied.
Michael Obafemi is back in training after recovering from injury, but the game comes too soon for him.
Will Smallbone and Oriol Romeu remain out.
Southampton XI: Forster; Walker-Peters, Bednarek, Vestergaard, Bertrand; Walcott, Ward-Prowse, Diallo, Armstrong; Ings, Redmond.
Road to the final
Both teams have faced several hard opponents on their way to Wembley.
Leicester started off their FA Cup campaign away to Stoke City in the Third Round.
Goals from James Justin, Marc Albrighton, Ayoze Perez and Harvey Barnes, ensured that the Foxes ran away 4-0 winners comfortably.
They then faced Brentford away in the Fourth Round.
An early goal from Mads Bech Sorensen meant that the Foxes would need to show really character against an impressive Brentford side. Second half goals from Cengiz Under, Youri Tielemans and James Maddison meant the Foxes came out 3-1 winners.
The Fifth Round saw the Foxes host Brighton and Hove Albion.
In what was a pretty dull affair and with extra time seemingly on the cards, with only seconds left of stoppage time, Kelechi Iheanacho was left unmarked in the box when a corner ball came in and he headed home.
The Quarterfinals saw the Foxes host Manchester United. An incredible performance with a goal from Tielemans and a brace from Iheanacho saw Leicester comfortably into the Semi-Finals and a trip to Wembley.
The Saints started off their campaign at home to League One side Shrewsbury Town.
Goals from Dan Nlundulu and James Ward-Prowse saw them safely through 2-0 winners.
They then hosted last year’s winners Arsenal in Round Four.
In what was an exciting tie between two evenly matched sides, an own goal from Arsenals Gabriel was the difference and saw Southampton into the Quarters.
The Quarterfinals saw the Saints travel along the south coast to Bournemouth where goals from Moussa Djenepo and a brace from Nathan Redmond saw them comfortably on their way to Wembley.
The two sides last met back in January when the Saints made the trip to the East Midlands.
In an entertaining game, Leicester took the lead after 37 minutes when Maddison fired a shot from a tight angle into the roof of the net to put the Foxes ahead.
Southampton came back fighting in the second half and had a couple of chances forcing Schmeichel into a couple a good saves.
But for all the Saints attacks trying to get back into the game they were left exposed and in stoppage time an old school Leicester counterattack saw Barnes through on goal before he slotted home to seal all three points and make it 2-0.
Speaking to the media before the game Brendan Rodgers spoke of the importance his players have the right mentality going into the game.
He said: “We want to arrive into the final, absolutely - but to do that we have to play well. It’s important we go in with the right energy and mentality. We’ve stayed calm, we give clarity to the week and we need to bring our A game”
Ralph Hasenhüttl spoke of how important he knows this game is to the fans of Southampton.
He said: "I mean, we are now allowed to walk around and go in the city, and the shops are open and you can be sure that people are coming to you and telling you how important it is for them to win this cup.
"You can feel and you can see why it is important for us, because it's the easiest way for us to win a trophy and this chance we have to take, and to show them how much we are driven like we have shown in the past rounds."
How to watch
The game Kicks-off at 18:00 GMT on Sunday evening.
The game will be broadcast on BT Sport 1.
Alternatively, you can follow all the live updates on the Vavel website and or Twitter page @LeicesterVAVEL.