Derbyshire have ended Sussex's promotion hopes.
Chasing a tough target of 345, Sussex folded against a depleted Derbyshire attack and were dismissed for 163 with Fynn Hudson-Prentice, Anuj Dal and Matt Critchley each taking three wickets.
The 181-run defeat - unthinkable after Sussex took big a first innings lead - means they cannot catch the top three in the division, with Northants' victory over Durham - completed on Thursday morning - confirming that Jason Gillespie's men will be in the second division again next season.
It was Sussex's failure to take wickets on day two as Derby wiped out their deficit and built up a big lead that cost the men from Hove dear.
Sussex head coach Jason Gillespie said: "We've just got to find a way to be better and that's the challenge we've set ourselves as individuals and as a team. They work really really hard at the game but we need to make better decisions with the bat, be more ruthless with the ball and rive home the advantage when we've got it which we did in parts in this game.
"We played well in patches but patches don't win you many games of first-class cricket. If you play consistently better than the opposition over a four day game, more often than not you will come out on top.
"If you have a good hour or a good session but then a poor session, it;s a lottery then and more often than not you will be on the wrong side of the ledger and that's something we've got to learn from."
Ollie Robinson had given them hope with 5 for 88 and Reece Topley claimed 4 for 58 after Derbyshire lost their last eight wickets for 77 to be bowled out for 437 but that was soon snuffed out as the home side celebrated with more than 24 overs left in the day.
The damage to Sussex’s prospects was inflicted on day two but at least the visitors responded with a vastly improved performance with the ball lead by Robinson.
He ran in with intent to take the first five wickets before Reece Topley polished off the tail to leave Sussex with a slim chance of pulling off the win they needed.
Their chase was made easier in theory by an injury to seamer Tony Palladino but Sussex could not put together a meaningful stand and came up well short.
Robinson showed what might have been if his side had bowled with discipline at the start of Derbyshire’s second innings when he had Billy Godleman caught at second slip off the first ball of the day.
Dal quickly followed, edging another that moved away late, and when Harvey Hosein fell across the crease and was lbw, the home side had lost three wickets in five overs.
Matt Critchley and Matt McKiernan quickly followed and after Leus du Plooy and Fynn Hudson-Prentice took Derbyshire’s lead towards 350, Topley completed a memorable return to championship cricket.
He pinned du Plooy lbw and tempted Hudson-Prentice into a pull to deep midwicket before an inswinging yorker accounted for Dustin Melton.
Sussex knew time was not a factor and the wicket had flattened out but although several players got a start, none of them could play the substantial innings needed.
Luke Wells was athletically caught at square leg by McKiernan and Will Beer played around his front pad to be lbw to Dal.
Salt had flirted with danger numerous times before he steered Dale to fist slip and Delray Rawlins fell to a loose shot for the second time in the match in the penultimate over before tea.
Sussex’s fate was settled when Hudson-Prentice had Stiaan Van Zyl and David Wieise caught behind off successive balls, leaving Ben Brown to watch helplessly as his side subsided meekly to the leg-spin of Critchley.