Match ends, Leeds United 2, Derby County 4.
Substitute Jack Marriott was the hero as Derby stunned Leeds to set up a Championship play-off final against Aston Villa after a wild night at Elland Road.
The hosts took the lead on the night to go 2-0 up on aggregate when Stuart Dallas tapped in after Liam Cooper's header had hit the post.
Marriott levelled with his first touch before half-time after a horrendous mix-up at the back between Cooper and keeper Kiko Casilla.
Frank Lampard's side turned the tie on its head after the break through Mason Mount and a Harry Wilson penalty, only for Dallas to drift in and fire Leeds back level on aggregate.
Leeds defender Gaetano Berardi was then sent off for a poor challenge on Bradley Johnson before Marriott won it late on with a composed finish.
There was still time for Derby defender Scott Malone to join Berardi in being dismissed after collecting a second booking in time added on.
For Leeds, who finished third in the table, defeat means their spell outside the top flight will stretch to a 16th year.
Derby, meanwhile, become the first team to lose the first leg of a Championship play-off semi-final at home and go on to reach the final meaning Lampard's first season in management will end at Wembley.
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Rams stage improbable comeback on incredible night
There had been no indication of what would transpire on this crazy night in West Yorkshire when Dallas put Leeds in front.
They had won all three of the previous meetings between the two sides this season by an aggregate of 7-1 and Derby looked short of ideas when they did manage to launch any attacks.
However, with half-time approaching, Lampard brought Marriott on for midfielder Duane Holmes and the former Peterborough man had an instant impact, tapping in after Cooper and Casilla ran in to one another to leave him with an open goal.
A brilliant improvised finish from Chelsea loanee Mount put them in front on the night just moments into the second half and Harry Wilson, on loan from Liverpool, kept his composure to fire home from the spot after Cooper pulled Mason Bennett's shirt in the area to put Derby ahead in the tie for the first time and spark wild scenes amongst the travelling Rams supporters.
Leeds responded well and Dallas deservedly pulled one back to draw them level at 3-3 on aggregate with a wonderful finish, but Berardi's red card shifted the momentum once more and defender Richard Keogh sprinted out of defence to play a measured pass for Marriott to poke in and book the Rams a trip to Wembley on Monday, 27 May.
Leeds fall short again
With four games of the regular season to go, Leeds had been in the driving seat to win automatic promotion after a hugely impressive season under veteran Argentine head coach Marcelo Bielsa.
However, they suffered a shock home defeat by strugglers Wigan on Good Friday before losing at Brentford three days later as their three-point lead over Sheffield United turned into a three-point deficit that they were unable to overturn.
With many fans fearing their moment had passed, they put in an assured performance at Pride Park in the first leg to come into this game with a one goal advantage but, once more, they could not finish the job.
Bielsa reflected that the defeat by Wigan that took their momentum away had been "the decision of God".
But that was only part of a hugely emotional campaign that saw him personally pay a £200,000 fine after the club were reprimanded for sending a member of staff to watch Derby train before the league game between the teams at Elland Road in January.
That 'Spygate' saga, which Derby boss Lampard described at the time as "not right", remained a key talking point before and during the play-off meetings.
And after beating Leeds for the first time this season, it was clearly at the forefront of the minds of Derby's players as they celebrated by pretending to hold binoculars over their eyes.
Will Bielsa stay or go?
After failing to take Leeds up, Bielsa now faces a huge decision of his own whether to carry on as boss and try to mount another promotion challenge next season.
Prior to joining Leeds on a two-year deal last June, the 63-year-old last completed a full season as a manager when he guided Marseille to fourth in Ligue 1 in 2014-15.
He was questioned about whether he would stay with Leeds in the immediate aftermath of his side's play-off disappointment but would not be drawn on where his future lay.
"If the club offers me the opportunity to carry on then I will consider the proposal," he said.
'I wanted this so badly' - post-match reaction
Derby boss Frank Lampard:
"I'd be naive to say I totally believed we could come back but I had belief in the players.
"As a manager, the pressure is more intense than as a player. I wanted this so badly, you worry you want it too badly. I'm very proud.
"I told Jack Marriott I thought he could have an impact in the game because he was disappointed not to be starting.
"We'll be underdogs in the final and tomorrow we start again."
Leeds head coach Marcelo Bielsa:
"It's a painful situation and I'm disappointed.
"We should have had one or two more in the first half and then the second half broke immediately.
"We lost control. We had 20 minutes without control and I couldn't find a solution."
- 28BerardiBooked at 78mins
- 6CooperBooked at 57mins
- 23PhillipsBooked at 27mins
- 19HernándezBooked at 20mins
- 43KlichSubstituted forClarkeat 86'minutes
- 9BamfordBooked at 31minsSubstituted forBrownat 88'minutes
- 5TomoriBooked at 5mins
- 46MaloneBooked at 90mins
- 23HolmesSubstituted forMarriottat 44'minutes
- 7WilsonSubstituted forColeat 90+3'minutes
- 8MountBooked at 65mins
- 10LawrenceBooked at 20mins
- 20BennettSubstituted forHuddlestoneat 59'minutes
- Anthony Taylor
- Shots on Target
Second Half ends, Leeds United 2, Derby County 4.
Foul by Isaiah Brown (Leeds United).
Jayden Bogle (Derby County) wins a free kick in the defensive half.
Attempt missed. Isaiah Brown (Leeds United) right footed shot from outside the box is too high from a direct free kick.
Substitution, Derby County. Ashley Cole replaces Harry Wilson.
Second yellow card to Scott Malone (Derby County) for a bad foul.
Pablo Hernández (Leeds United) wins a free kick in the attacking half.
Foul by Scott Malone (Derby County).
Attempt saved. Jack Clarke (Leeds United) right footed shot from the right side of the box is saved in the centre of the goal.
Attempt saved. Stuart Dallas (Leeds United) right footed shot from outside the box is saved in the top right corner. Assisted by Isaiah Brown.
Attempt saved. Liam Cooper (Leeds United) header from the centre of the box is saved in the bottom left corner. Assisted by Pablo Hernández with a cross.
Substitution, Leeds United. Isaiah Brown replaces Patrick Bamford.
Luke Ayling (Leeds United) wins a free kick on the right wing.
Foul by Mason Mount (Derby County).
Jack Harrison (Leeds United) wins a free kick in the attacking half.
Foul by Harry Wilson (Derby County).
Substitution, Leeds United. Jack Clarke replaces Mateusz Klich.
Goal! Leeds United 2, Derby County 4. Jack Marriott (Derby County) right footed shot from the left side of the box to the centre of the goal. Assisted by Richard Keogh.
Harry Wilson (Derby County) hits the left post with a left footed shot from the centre of the box. Assisted by Mason Mount with a through ball.