Match ends, Brighton and Hove Albion 1, Derby County 1.
Brighton's draw with Derby means they must beat Middlesbrough on Saturday to be promoted to the Premier League.
Both clubs will aim to join Burnley in the top tier after they secured a top-two finish by beating QPR on Monday.
James Wilson netted a scrappy 95th-minute equaliser for Brighton, who also had Lewis Dunk sent off after Andreas Weimann had put Derby in front.
Brighton are third, level on points with Boro, ahead of their trip to the Riverside on the season's final day.
A defeat by play-off-bound Derby would not have altered Brighton's promotion prospects too much, given they have an inferior goal difference to Middlesbrough.
Chris Hughton's side will go to Teesside without suspended centre-back Dunk, who was sent off with seven minutes remaining for two yellow cards - the second of them coming after a reckless lunge on Craig Bryson.
Anthony Knockaert came closest to opening the scoring for Brighton against the Rams, his first-half free-kick being headed off the line by Jason Shackell.
But Derby looked the more likely of the two sides to find the net, particularly after half-time, and substitute Weimann fired in at the second attempt after David Stockdale had saved his initial shot.
However, Manchester United loanee Wilson turned in the equaliser after a goalmouth scramble from a corner to extend Brighton's unbeaten run to 13 games.
A point for the visitors means they cannot be caught by sixth-placed Sheffield Wednesday, and Darren Wassall's Rams now know they will face Hull City in the play-offs.
Brighton manager Chris Hughton: "We will be going to the Riverside with a winning mentality and we need to show enough quality.
"There's no doubt Middlesbrough will be the favourites and they'll be pleased with our result, but we've given ourselves every chance.
"We have been good away from home for the majority of the season and we haven't lost today. It is a big task but not one that is impossible.
"You don't have it all your own way over the course of the season... you have to take some blows and you get used to bouncing back."
Derby head coach Darren Wassall: "Brighton are having a magnificent season but our performance spoke volumes.
"We came and put on a fantastic display. We handled the occasion magnificently and I'm proud of the team.
"We want to keep momentum going into the play-offs and, credit to the squad, we have continuity building."
On their upcoming play-off clash with Hull: "Why not get promotion via the play-offs? We have as good a chance as anyone.
"We have beaten Hull twice already this season but Steve Bruce is a wily old fox. We are ready and we can't worry about anyone else."
- 5DunkBooked at 83mins
- 38SkalakSubstituted forLua Luaat 67'minutes
- 9BaldockSubstituted forWilsonat 67'minutes
- 30Lua Lua
- 2ChristieBooked at 72mins
- 4BrysonSubstituted forButterfieldat 90+1'minutes
- 34ThorneBooked at 38mins
- 19HughesSubstituted forBuxtonat 90+4'minutes
- 23InceBooked at 63minsSubstituted forWeimannat 70'minutes
- Robert Madley
- Shots on Target
Second Half ends, Brighton and Hove Albion 1, Derby County 1.
Goal! Brighton and Hove Albion 1, Derby County 1. James Wilson (Brighton and Hove Albion) right footed shot from the centre of the box to the top left corner following a corner.
Substitution, Derby County. Jake Buxton replaces Will Hughes.
Corner, Brighton and Hove Albion. Conceded by Jason Shackell.
Attempt blocked. James Wilson (Brighton and Hove Albion) left footed shot from outside the box is blocked. Assisted by Tomer Hemed.
Delay over. They are ready to continue.
Substitution, Derby County. Jacob Butterfield replaces Craig Bryson because of an injury.
Delay in match Craig Bryson (Derby County) because of an injury.
Foul by Will Hughes (Derby County).
Beram Kayal (Brighton and Hove Albion) wins a free kick in the defensive half.
Attempt missed. Chris Martin (Derby County) right footed shot from outside the box is too high from a direct free kick.
Second yellow card to Lewis Dunk (Brighton and Hove Albion) for a bad foul.
Craig Bryson (Derby County) wins a free kick in the attacking half.
Foul by Lewis Dunk (Brighton and Hove Albion).
Cyrus Christie (Derby County) wins a free kick in the defensive half.
Foul by Beram Kayal (Brighton and Hove Albion).
Delay over. They are ready to continue.
Delay in match Johnny Russell (Derby County) because of an injury.
Delay in match Liam Rosenior (Brighton and Hove Albion) because of an injury.