Match ends, Chelsea 2, West Bromwich Albion 5.
Matheus Pereira and Callum Robinson scored twice as struggling West Brom overwhelmed 10-man Chelsea and emphatically ended the 14-match unbeaten start to Thomas Tuchel's tenure at Stamford Bridge.
The Baggies were as brilliant as the Blues, who had Thiago Silva sent off in the 29th minute, were abject.
The result moves the 19th-placed side to within seven points of 17th-placed Newcastle, and even if they do not manage the improbable and escape relegation, they will look back on this day with great pride.
Tuchel said he "did not see the second half coming". His side took the lead through Christian Pulisic in the 27th minute, but then lost Thiago and conceded twice in first-half stoppage time before West Brom inflicted further damage after the break.
"There is no need to concede five goals when you have our quality," he told BBC Sport. "We are all in on that - myself included.
"We could not adapt to the situation, which is a surprise as we were leading. We are normally confident enough to defend, but we were sloppy and kept making big mistakes. It's tough to digest.
"I did not see that second half coming. We tried to give a bit of security with a back three, but we were never in a flow. We had some chances but we didn't defend well."
Two minutes after Pulisic was quickest to react when Marcos Alonso's free-kick came off the post, Thiago was shown a second yellow for a rash challenge on Okay Yokuslu.
That was followed by a horror show in first-half stoppage time as Pereira took centre stage. The Brazilian's first effort was a cute finish over Edouard Mendy before he finished off a gorgeous move with a low strike between three defenders. In between those goals, Matt Phillips produced a strike that came off the woodwork.
West Brom then went for the jugular after the break as substitute Robinson made it 3-1 with a sensational side-foot volley before Mbaye Diagne finished off a sweet move with an even sweeter strike.
England midfielder Mason Mount, brought on at half-time, tapped in to reduce the deficit but the Baggies resisted other Chelsea threats before Robinson capped off a great day for the visitors with a clipped finish.
"I am delighted - not only with the win, but the style," Allardyce said.
"It has been coming for a while, but our lack of finishing has cost us. What we saw today is what we have been missing. The quality of our goals and our football brought us a fantastic result. We were desperate for it.
"We started really well, we pressed and didn't sit back. I had three weeks to think about this game and sort the tactics. We said, 'Let's go, let's go challenge them' and it worked extremely well."
Sluggish Chelsea severely punished
Was this Thomas Tuchel's first major tactical misjudgement?
His reasons for leaving some first-team regulars from the first XI, including Mount and Kai Havertz, was because he wanted the freshest legs on the pitch because of the extra exertions of his benched players during the international break.
Those who started might have been sharper physically, but they were mentally sluggish at times with Thiago paying the biggest price.
It was a risk to start the 36-year-old Brazilian, who had just returned from two months out with an injury. He took 'one for the team' when he pulled back Pereira on the edge of the area, but his second booking was inexcusable for a player of his stature.
Dane Andreas Christensen, who had deputised for Thiago, came on as midfielder Hakim Ziyech made way.
Striker Timo Werner came into the match short of confidence and having missed a 'sitter' for Germany against North Macedonia. He probably wished he reacted as quickly as Pulisic did for the Blues goal, although he did set up Mount's goal.
By that stage, both he and his team-mates were probably wishing they were still on international duty.
A side who had conceded only two goals in Tuchel's previous 14 games were outwitted and embarrassed by one who had struggled to score this season.
Chelsea have four days to recover before they face Porto in the Champions League quarter-finals.
West Brom attack dazzle at Stamford Bridge
This was the perfect storm for West Brom - a day when their players were on song in contrast to their counterparts.
Pereira has been one of the shining lights for the Baggies during this difficult season, and even though he only signed a permanent deal last summer it might prove difficult to hang on to him if they go down.
The timing of his run for the first goal from goalkeeper Sam Johnstone's punt downfield was perfection, as was his clinical finish for the second.
Robinson, who only came on in the 37th minute for the injured Branislav Ivanovic, immediately adapted to the momentum of his team. His finish for West Brom's third was the pick of the bunch.
Allardyce has never experienced relegation from the Premier League during his long managerial career - if he maintains that record this season then it will arguably be his greatest feat.
Hat-trick for Allardyce - stats
- Chelsea have conceded eight goals in their two games against West Brom in the Premier League this season, the most they have shipped against one side in a single campaign in the competition.
- Chelsea conceded five-plus goals in a home Premier League game for only the second time, last doing so against Arsenal in October 2011 (3-5).
- Allardyce is the first manager to win away at Stamford Bridge in the Premier League with three different clubs (Bolton Wanderers, Crystal Palace, West Brom).
- Chelsea conceded at home under Tuchel for the first time in all competitions, in the Blues' eighth game at Stamford Bridge with the German coach in charge.
- Pulisic's opener for Chelsea was his first for the Blues in his last 21 appearances in all competitions, since netting against Leeds in December in the Premier League.
- Thiago's red card on 29 minutes was Chelsea's earliest in the Premier League since August 2017 when Gary Cahill was sent off against Burnley after just 14 minutes.
- Johnstone is the first goalkeeper to provide a Premier League assist for West Brom and just the second to do so this season, after Ederson for Manchester City v Tottenham in February.
- Pereira's two goals for West Brom came just two minutes and 46 seconds apart, with these the Brazilian's first goals in nine league games.
- Robinson is the first player in Premier League history to score at least twice in home and away games against Chelsea within a single season.
Chelsea are at Porto on Wednesday in their Champions League quarter-final first leg (20:00 BST) before they travel to Crystal Palace next Saturday (17:30). West Brom host Southampton in the league, a week on Monday (18:00).
- 6Thiago SilvaBooked at 29mins
- 5JorginhoSubstituted forHavertzat 72'minutes
- 17KovacicBooked at 66mins
- 22ZiyechSubstituted forChristensenat 33'minutes
- 10PulisicSubstituted forMountat 45'minutes
- 27O'SheaSubstituted forIvanovicat 24'minutesSubstituted forRobinsonat 37'minutes
- 10PhillipsSubstituted forLivermoreat 89'minutes
- 12Costa Pereira
- 17DiagneBooked at 14mins
- David Coote
- Shots on Target
Second Half ends, Chelsea 2, West Bromwich Albion 5.
Mateo Kovacic (Chelsea) wins a free kick in the defensive half.
Foul by Jake Livermore (West Bromwich Albion).
VAR Decision: Goal West Bromwich Albion - Chelsea 2-5 West Bromwich Albion (Callum Robinson).
Goal! Chelsea 2, West Bromwich Albion 5. Callum Robinson (West Bromwich Albion) right footed shot from the right side of the box to the bottom left corner. Assisted by Matheus Pereira with a through ball.Goal confirmed following VAR Review.
Substitution, West Bromwich Albion. Jake Livermore replaces Matt Phillips.
Foul by Marcos Alonso (Chelsea).
Ainsley Maitland-Niles (West Bromwich Albion) wins a free kick in the defensive half.
Attempt blocked. Mbaye Diagne (West Bromwich Albion) right footed shot from outside the box is blocked. Assisted by Callum Robinson.
Foul by Mason Mount (Chelsea).
Ainsley Maitland-Niles (West Bromwich Albion) wins a free kick in the defensive half.
Offside, West Bromwich Albion. Kyle Bartley tries a through ball, but Matt Phillips is caught offside.
Foul by Reece James (Chelsea).
Callum Robinson (West Bromwich Albion) wins a free kick in the defensive half.
Attempt missed. Mason Mount (Chelsea) right footed shot from outside the box misses to the left. Assisted by Kai Havertz.
Attempt blocked. Timo Werner (Chelsea) right footed shot from outside the box is blocked. Assisted by Andreas Christensen.
Attempt missed. Okay Yokuslu (West Bromwich Albion) header from very close range is just a bit too high. Assisted by Matheus Pereira with a cross following a corner.
Corner, West Bromwich Albion. Conceded by Kurt Zouma.
Attempt missed. Kurt Zouma (Chelsea) left footed shot from the centre of the box is too high. Assisted by Timo Werner with a headed pass following a corner.
- Dragons' Den returns: A new series of eager entrepreneurs hoping to strike the deal of a lifetime
- Your film night sorted: Epic films for you to choose from this Easter weekend