The first leg will take place at Bramall Lane on Saturday with the return fixture at the City Ground three days later.
Whoever progresses over the two-legged tie will play either Huddersfield Town or Luton in the final at Wembley on Sunday 29 May.
Forest finished one place and five points above United in the final league table.
While the Blades have failed to win promotion via the play-offs in no fewer than eight attempts, manager Paul Heckingbottom has previously tasted success.
He twice won promotion via the end-of-season contest as a player and was promoted with Barnsley as caretaker manager in the 2015/16 season.
Here’s everything you need to know ahead of Saturday’s first leg.
Sheffield United vs Nottingham Forest will be shown on Sky Sports Football.
Coverage begins at 2.45pm.
Who is the referee?
Experienced Premier League ref Andre Marriner is the man in the middle with Simon Long and Richard West his assistants. The fourth official is Kevin Friend.
Sheffield United vs Nottingham Forest team news
Iliman Ndiaye and Morgan Gibbs-White are likely to once again lead the line for Sheffield United.
A question mark remains over Billy Sharp’s fitness.
The influential Blades skipper and 15-goal top-scorer missed the last two games of the season with a calf injury.
Nottingham Forest captain Lewis Grabban, who has scored 12 Championship goals this season, is out for the rest of the season because of a hamstring injury.
Keinan Davis also missed the end of the regular season through injury, but it is unknown whether he may return for the play-offs now Forest’s campaign has been extended.
Forest regulars Steve Cook and Ryan Yates missed the final day against Hull but could also return against United.
Sheffield United vs Nottingham Forest odds
Sky Bet are offering the following odds on Sheffield United vs Nottingham Forest:
Sheffield United: 5/4
Nottingham Forest: 12/5