The Rams won 2-0 on the night at the KC, but they were left to pay after losing so heavily to Steve Bruce’s side on Saturday.
County came flying out the traps and they looked like they meant business as Johnny Russell opened the scoring after seven minutes.
A dangerous cross was sent into the box on 36 minutes and Hull defender Andrew Robertson put the ball into his own net from six yards.
The east Yorkshire crowd were on edge as Derby went for a third, but it eluded them and the Tigers are one match away from a Premier League return.
The play-off final will take place at 5pm on Saturday, May 28.
Knowing the odds were stacked against them, Derby began at a high tempo and took the lead when Hull failed to deal with a cross to the back post from Cyrus Christie.
The ball was headed down by Chris Martin and Russell saw his first shot blocked but forced home the rebound from a couple of yards.
Ten minutes later Russell curled a free-kick just wide as Hull continued to soak up pressure and it took 25 minutes for the home side to have a shot when Jake Livermore fired well over.
With the break less than ten minutes away, Derby got their second when a low cross from the left by Andreas Weimann was met by Robertson and sent past Eldin Jakupovic.
Hull had a penalty appeal turned down when Moses Odubajo went to ground and the same player saw his shot deflected wide for a corner as Derby continued to be solid at the back.
With the tie finely balanced both sides began the second half in search of the next goal, Derby being denied in a goalmouth scramble by some hurried Hull defending.
Derby looked composed in midfield but were not quite able to make the most of their possession, Will Hughes firing over after one period of concerted pressure and Martin then tested Eldin Jakupovic with a shot which the Hull keeper beat away with his fists.