Only Matt Phillips’ goal separated the two sides after a tight first 45 minutes, but in a brilliant second half showing, the Rams responded in herculean fashion to stun their opponents in their own backyard.
Darren Bent’s fine strike from 20-yards out got them back on level terms, but attacking towards a packed out away end, Tom Ince’s ninth goal of the season saw Steve McClaren’s side turn the game on its head to defeat the Baggies and secure their place in the fourth round of the competition.
Ahead of this meeting, McClaren, the Derby boss, had called for his side to show a reaction.
Earlier in the week his side had been humbled by Norwich City in East Anglia on a luckless afternoon and backed by a sell-out 5,351 away crowd, Derby did that.
They enjoyed a bright start at the Hawthorns, enjoying the larger spells of possession without seriously threatening Boaz Myhill’s goal.
It could have been different. A superb goalline clearance from Richard Keogh prevented Salomon Rondon from breaking the deadlock on four minutes after the forward had seized on a rare error from Scott Carson.
McClaren’s men also dealt with a series of dangerous corners launched into the area as the physical hosts enjoyed a brief spell with a stranglehold, but at the other, Derby looked a threat themselves.
Abdoul Camara, making only his second start of the season, particularly looked a menace and a key player for the visitors and showed great pace to collect an inch-perfect diagonal from Julien de Sart, who was making his debut. Camara scampered away, before cutting the ball back to Tom Ince, who in turn left the ball from Craig Bryson who fired an effort over the crossbar.
Enjoying a lot of joy down the left, Camara then forced Myhill into a save at his near post with a deflected cross shot, but at a stage when very little separated the Rams from their Premier League opponents, a goal from a man in the form of his life opened the scoring.
Slack in possession deep inside their own territory, Derby gifted Phillips the ball and with time, the wideman made the most of the opportunity by curling an effort beyond the reach of Carson and into the corner of the goal.
Tony Pulis’ side ended the half strongly and had chances to double their lead as James Morrison fired over and Phillips saw another effort blocked after cutting inside Hanson down the right.
The Rams survived, however, and with the words of their manager ringing in their ears responded quickly in the second half to stun their Premier League opponents and turn the game on its head.
With even more belief and fight than they showed in the first half, Bent got the ball rolling after he was afforded time to shoot from 20 yards out and he did so with aplomb, smacking a high effort out of the reach of Myhill to send the away at the Hawthorns into raptures.
It got better just three minutes later. Ince, a constant menace to the West Brom backline in the second half, took this grand stage by the scruff of the neck and stunned all four corners of the Hawthorns by curling a sublime free kick from the edge of the area over the wall into the far corner of Myhill’s net.
After the post came to Derby’s rescue in this pulsating half, McClaren’s men picked up where they left off and could so easily have added a third as the clock ticked down.
Myhill came to the Baggies’ rescue on two occasions – firstly denying Ince, whilst he also, somehow, clawed away a Bent effort after he swivelled and smacked a low shot towards the near corner.
Naturally, the final 10 minutes were chaotic as West Brom piled pressure on Carson’s goal in a desperate search for a late equaliser, but it was to no avail - their chances more often flying over or wide - but there was one final chance for Rondon, but Carson was alert to tip behind his header with a fine instinctive save deep into five minutes of added time.
The final whistle brought a huge roar from the away end for a third time this afternoon, confirming one of the day’s standout FA Cup results. What did it mean to the players? Most dropped to their knees and who can blame them. The sheer effort that was put into winning this game was phenomenal and one thing’s for sure, this is a day that will be enjoyed for some time
West Bromwich Albion: Myhill; Dawson, McAuley, Olsson (Chadli, 78), Galloway (Robson-Kanu, 68); Yacob, Fletcher (C); Phillips, Morrison, McClean; Rondon
Substitutes not used: Foster; Gardner, Harper, Field, Wilson
Derby County: Carson, Baird, Keogh (C), Shackell, Hanson (Pearce, 82); de Sart, Bryson (Vernam, 89), Johnson; Ince, Camara (Vydra, 86), Bent
Substitutes not used: Mitchell, Russell, Weimann, Guy