Two stunning Derby strikes in the space of three second-half minutes dumped Albion out of the FA Cup, reports Matt Wilson at The Hawthorns.
Matt Phillips had given the Baggies a deserved lead 10 minutes before half-time with a clinical finish following a Richard Keogh mistake.
But Darren Bent equalised against the run of play in the 51st minute with a stunning strike from the edge of the box before Tom Ince won it three minutes later from a free-kick.
Tony Pulis made four changes, handing Everton loanee Brendan Galloway his first start since September and bringing Jonas Olsson in for the injured Jonny Evans.
James McClean got a well-earned start ahead of the out-of-form Nacer Chadli, and he switched Boaz Myhill for Ben Foster in goal.
But it was Derby's number one Scott Carson who nearly handed Albion the lead in the fourth minute, when he kicked the ball straight to Salomon Rondon.
It was a howler from the former Albion keeper on his old stomping ground, and only went unpunished thanks to a superb block on the line from captain Keogh.
Derby started the game lively, both on the pitch and in the stands, but Albion were threatening from set-pieces and were causing problems in the County box.
And the hosts started to see more of the ball as the game settled down, pressing the visitors high and forcing them into mistakes.
It paid off in the 35th minute, when James McClean's energy forced Keogh into a mistake on the edge of the box and he gave the ball straight to Phillips.
The winger still had plenty to do though, and he drove towards goal with purpose, feinting to shoot with his left before curling a clinical finish past Carson with his right.
It could have been two just minutes later when the goalscorer surged down the right flank and beat three defenders before pulling the ball back.
James McClean stepped over it and James Morrison manouvered himself into a shooting position before shooting over.
Derby's calamitous defending continued after the break, and Rondon outmuscled Bradley Johnson on the edge of the box before shooting wide. And Albion were made to pay for those wasted chances.
Not long after, Bent struck an unstoppable shot from the edge of the box that drew the Rams level. It was a bolt from the blue but it woke the visitors up and three minutes later they were ahead.
When the in-form Ince burst down the right he was shoved over by James Morrison 25 yards out before he could get his shot away, but the former Blackpool winger got up and dusted himself down before curling the free-kick into the bottom corner.
Boaz Myhill was rooted and the Baggies were stunned. They had been on top for most of the game but now they were losing.
Craig Dawson nearly drew them level just after the hour mark but his header at the back stick hit the post and bouncd clear.
In an effort to avoid defeat, Pulis brought Hal Robson-Kanu and Nacer Chadli on for two defenders, but even though the Baggies piled on the pressure, they were unable to find an equaliser.
There was arguably more riding on this game than any other so far this season.
Lose a league match and you can recover a week later, but lose in the FA Cup, and you have to wait a whole year before your next shot.
With relegation a distant concern and the Baggies on red-hot form, particularly at home, where
they had won four out of the last five, this was the perfect chance to go deep in the Cup.
The second-largest Hawthorns crowd of the season turned up, and even though that was buoyed by Derby's 5,200-strong support in the Smethwick End and Albion's competitive ticket pricing, it still proved just how important today's game was to the fans.
Before kick-off the feeling was a mix of excitement tinged with dread because recently, dreams of Cup glory have been snuffed out early.
This season's limp exit in the League Cup at Northampton Town followed last season's stuttering FA Cup run that ended in a dark day at the Madejski Stadium as Albion weren't only knocked out by lower-league opposition, but a so-called supporter struck Chris Brunt with a coin.
Fast-forward a year and the mood couldn't be different. Tony Pulis has won round the fans and there is confidence coursing through the team.
But the fans were right to be concerned. Albion were cruising thanks to Matt Phillips's fifth goal of the season and they were pressing Derby high and forcing them into mistakes.
Fletcher, McClean, Morrison and Phillips all flooded forward with intensity and it was McClean's energy that forced Richard Keogh into a mistake for Phillips's goal.
For some reason Derby continued to play it out from the back and the Premier League side should have been two or three up by the time Darren Bent equalised, but they were punished for wasting their chances.
Bent's was against the run of play, but it sparked County into life and three minutes later they led.
Ince's free-kick was superb but there will be question marks over Boaz Myhill, who remained rooted to the spot as the ball flew inside his near post.
In the past four years Wigan, Hull, Aston Villa, and Crystal Palace have all reached the final. Pulis did it with Stoke in 2011, and he wanted to do it with Albion this season.
The Baggies huffed and they puffed, managing 23 shots and 15 corners during the game, and even finishing with more than 50 per cent possession. But it wasn't to be.
Albion are still going well in the league, and are on course to break the 50-point barrier for the first time in the Premier League. But that will be of little comfort tonight.
03 Huge chance for Albion when Scott Carson kicks the ball straight to Rondon, but a superb block on the line from Richard Keogh denies him.
35 GOAL ALBION - McClean's pressing forces Derby into a mistake and the ball falls to Matt Phillips 35 yards out. He drives towards goal and curls a clinical finish past Carson.
36 Surging run down the flank from Phillips, and he pulls it back. McClean steps over it, before Morrison shoots just over.
51 GOAL DERBY - Darren Bent equalises with a stunning strike from the edge of the area.
54 GOAL DERBY - Tom Ince gives Derby the lead with a free-kick curled into the bottom corner that leaves Boaz Myhill rooted.
62 Craig Dawson hits the post with a header at the back stick.
90 Nacer Chadli clips a ball into the box but Carson tips Rondon's header over the bar
Man of the match
Matt Phillips - Was superb again and didn't deserve to be on the losing side.
Albion (4-2-3-1): Myhill, Dawson, McAuley, Olsson (Chadli 78), Galloway (Robson-Kanu 67), Fletcher (c), Yacob, Phillips, Morrison, McClean, Rondon. Unused subs: Foster (gk), Wilson, Field, Gardner, Harper, Robson-Kanu.
Derby County (4-3-3): Carson, Barid, Keogh (c), Shackell, Hanson (Pearce 81), Bryson (Vernam 88), De Sart, Johnson, Ince, Bent, Camara (Vydra 86). Unused subs: Subs: Mitchell (gk), Guy, Russell, Weimann.
Referee: Jonathan Moss
Attendance: 25,288 (5,200 Derby)Subscribe to our Newsletter