West Brom centre-back Semi Ajayi had earlier hit the post, before scoring the winner
Semi Ajayi's 94th-minute tap-in saw West Bromwich Albion snatch a fourth consecutive Championship victory and break the hearts of a dogged Peterborough side.
The Baggies had edged a scrappy but high-energy game - Ajayi hitting the post earlier in the second half after home keeper Christy Pym's excellent save from debutant Jordan Hugill's header had fallen to the centre-back - but Posh largely stood up to the aerial bombardment from the visitors.
However, Valerien Ismael's side kept plugging away and deep into five minutes of stoppage time Matt Phillips powered his way to the byline and fizzed the ball across goal for Ajayi to divert in unmarked.
The goal sparked jubilant scenes and sees West Brom join leaders Fulham on 13 points, while managers Ismael and Darren Ferguson clashed as tempers flared after the full-time whistle.
For Posh it is the second time they have been on the wrong end of last-gasp drama at the Weston Homes Stadium this season, having conceded a 96th-minute equaliser to Cardiff earlier this month, and the newly-promoted side sit 20th with four points from their first five matches.
- Relive West Brom's dramatic win at Peterborough
Karlan Grant saw a low shot across goal parried away by Pym in a low-key first half, with the game coming to life after the introduction of on-loan Norwich striker Hugill before the hour mark.
He gave the Baggies another target for their countless corners and long throws and their persistence eventually paid off, though neutrals could not help but feel for a Peterborough side who had given their all and held their own as underdogs.
Ferguson's side posed an occasional threat on the counter, Jack Marriott shooting straight at Sam Johnstone when well-placed and Sammie Szmodics denied a golden chance by a crucial Dara O'Shea clearance.
There was a comical element to the match as West Brom's long-throw specialist Darnell Furlong was booked in the second half for taking too long over his efforts, as he regularly used a hoodie from a fan in the away end to dry the ball in the absence of any towels.
But, with three-goal striker Callum Robinson missing after testing positive for Covid-19 and having lost 6-0 to Arsenal in the Carabao Cup in midweek - albeit having made 11 changes - the Baggies will be delighted to have ground out an ugly away win.
At the end the managers exchanged words, failing to shake hands as they aggressively gesticulated towards each other, typifying the grit shown on the pitch by their respective sides during the 90 minutes.
Spot the difference - midway through the second half Ajayi hit the outside of the post from a similar position to where he would go onto score the winner
Peterborough boss Darren Ferguson told BBC Radio Cambridgeshire:
"I'm disappointed but I felt the players were magnificent to a man - I thought we stood up to them and went toe-to-toe with them all the way until the 94th minute.
"I can't do anything but compliment my players' performance.
"I didn't think they had too many clear-cut chances in the game - I thought we dealt with everything they threw at us.
"In the second half I thought when things opened up a bit we just picked the wrong pass every now and then - but as the game wore on there were spaces opening up and I felt we could go and nick it
"It was cruel for my players because they didn't deserve to lose that game."
West Brom manager Valerien Ismael told BBC Radio WM:
"This is what I love to see about football, the fans back inside the stadium, a last-minute goal.
"They're the best in football for sure, they create such positive and negative emotions and I was taken back to the same emotions I had as a player after that goal went in.
"I want to congratulate the players on the win, we had a really difficult week to prepare, a lot of injuries to manage and a Covid case.
"But we always stayed focused on our purpose and we managed the challenges well, showed a great mentality.
"We know we can score at any time and the learning from tonight is that it's important to stay strong and believe in ourselves right to the final whistle."
- Line-upsMatch StatsLive Text
Line-upsPeterborough
Formation 3-4-1-2
1Pym4Thompson6Kent3Butler23Ward18Norburn11Grant16Burrows7Szmodics9Clarke-Harris14Marriott
- 1Pym4ThompsonBooked at 71mins6Kent3Butler23Ward18NorburnBooked at 8minsSubstituted forTaylorat 45'minutes11GrantSubstituted forKnightat 81'minutes16Burrows7Szmodics9Clarke-Harris14MarriottSubstituted forDembéléat 59'minutes
Substitutes
- 2Edwards8Taylor10Dembélé12Knight21Tomlinson26Randall28Blackmore
West Brom
Formation 3-4-3
1Johnstone6Ajayi5Bartley4O'Shea2Furlong8Livermore27Mowatt3Townsend11Diangana18Grant10Phillips
- 1JohnstoneBooked at 64mins6Ajayi5BartleySubstituted forReachat 45'minutes4O'SheaBooked at 90mins2FurlongBooked at 82mins8Livermore27MowattBooked at 70mins3Townsend11DianganaSubstituted forHugillat 57'minutes18Grant10Phillips
Substitutes
- 17Hugill20Reach23Snodgrass29Gardner-Hickman33Taylor36Palmer41Fellows
Referee:James LiningtonMatch StatsHome TeamPeterboroughAway TeamWest BromPossessionHome47%Away53%ShotsHome5Away19Shots on TargetHome1Away3CornersHome6Away5FoulsHome9Away10Live TextPost update
Match ends, Peterborough United 0, West Bromwich Albion 1.
90'+6'Full Time
Second Half ends, Peterborough United 0, West Bromwich Albion 1.
90'+5'Booking
Dara O'Shea (West Bromwich Albion) is shown the yellow card.
90'+4'goalGoal!
Goal! Peterborough United 0, West Bromwich Albion 1. Semi Ajayi (West Bromwich Albion) right footed shot from very close range to the centre of the goal. Assisted by Matt Phillips.
90'+2'Post update
Jordan Hugill (West Bromwich Albion) wins a free kick in the defensive half.
90'+2'Post update
Foul by Jack Taylor (Peterborough United).
90'+1'Post update
Attempt missed. Jordan Hugill (West Bromwich Albion) left footed shot from the centre of the box is high and wide to the left. Assisted by Conor Townsend with a cross.
89'Post update
Offside, Peterborough United. Nathan Thompson tries a through ball, but Siriki Dembélé is caught offside.
83'Post update
Attempt missed. Jordan Hugill (West Bromwich Albion) header from very close range is high and wide to the left. Assisted by Matt Phillips with a cross following a corner.
82'Post update
Corner, West Bromwich Albion. Conceded by Jonson Clarke-Harris.
82'Booking
Darnell Furlong (West Bromwich Albion) is shown the yellow card.
81'Substitution
Substitution, Peterborough United. Josh Knight replaces Jorge Grant.
79'Post update
Offside, West Bromwich Albion. Darnell Furlong tries a through ball, but Matt Phillips is caught offside.
78'Post update
Attempt blocked. Jordan Hugill (West Bromwich Albion) header from the centre of the box is blocked. Assisted by Alex Mowatt with a cross.
77'Post update
Corner, Peterborough United. Conceded by Adam Reach.
74'Post update
Foul by Conor Townsend (West Bromwich Albion).
74'Post update
Jonson Clarke-Harris (Peterborough United) wins a free kick in the attacking half.
73'Post update
Offside, Peterborough United. Joe Ward tries a through ball, but Sammie Szmodics is caught offside.
73'Post update
Attempt blocked. Joe Ward (Peterborough United) right footed shot from outside the box is blocked.
72'Post update
Corner, Peterborough United. Conceded by Semi Ajayi.
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Source: bbc.com