LUKE Moore's deadly first-half double set Albion on their way to back-to-back Coca-Cola Championship wins - but the Baggies made hard work of it in this five-goal thriller.
The ex-England Under-21 striker opened his account for the season in the third minute and doubled his tally 31 minutes later as Roberto Di Matteo's men took complete control.
Chris Brunt's first goal of the campaign in the 41st minute appeared to have made the points safe even at this early stage.
But Craig Mackail-Smith's header in first-half stoppage time gave Posh a lifeline.
And within eight minutes of the restart, they really piled the pressure on Albion when Aaron Mclean netted.
The hosts spent most of the second period camped in the visitors' half, with the occasional Baggies threat on the break.
But Albion stood up to the test and breathed a huge sigh of relief when the final whistle was blown.
The victory was only marred by the sending-off of the impressive Graham Dorrans deep into stoppage time for a second bookable offence.
Loan signing Shaun Cummings was handed his Baggies debut while Marek Cech and Chris Wood earned recalls as Di Matteo made three changes.
The men to make way were the injured Gianni Zuiverloon (hamstring), Jonathan Greening, who was left out altogether, and the benched Robert Koren.
The Baggies made a lightening start by scoring from their first attack in the third minute.
Brunt challenged with Tom Williams for Shelton Martis' aerial pass and the ball rebounded off Gabriel Zakuani to Moore.
The former Villa striker controlled and unleashed a low shot from the edge of the area that squirmed under Posh keeper Joe Lewis and crept over the goalline.
Lewis got himself in a pickle again five minutes later when he came rushing off his line and almost gifted possession to Moore on the edge of the area - but Charlie Lee managed to clear the danger.
The visitors were making all the play and Dorrans' defence-splitting pass in the 12th minute put Brunt clean through.
The Northern Ireland international had plenty of time and opted to try and chip Lewis, only to see the ball drift wide of the right-hand post.
United were forced into a change in the 15th minute as Williams limped off.
Russell Martin came on at right-back while Lee switched to left-back.
The hosts were unfortunate not to equalise from a corner in the 25th minute as captain Scott Carson first saved at point-blank range from Mackail-Smith.
George Boyd then appeared to hit the right-hand post before Mclean saw his follow-up shot deflected just wide of the same upright.
Posh were enjoying a mini purple patch and, four minutes later, Craig Morgan headed Boyd's right-wing corner back across goal and Mackail-Smith missed a golden opportunity to level when he headed inches wide of the far post, with Carson beaten.
Back came Albion and Moore did well to stretch and get any sort of touch on Dorrans' right-wing cross in the 33rd minute, although he could only direct a tame effort at Lewis.
But just a minute later, substitute Martin gave the ball away to Cech who found Wood inside and he slid Moore in, with the striker producing the coolest of finishes to Lewis' left.
The Baggies extended their lead further in the 41st minute.
Moore showed tight, close control on the edge of the area and did well to play Cummings in behind the defence.
The 20-year-old full-back showed good composure and crossed into the six-yard box where the ball broke loose from Wood's challenge with Lewis and Brunt smashed home with his right foot from ten yards.
That appeared to be that.
But Darren Ferguson's men gave themselves a lifeline in the first minute of stoppage time when Mackail-Smith slid in at the back post from Shaun Batt's left-wing cross to convert with a diving header.
Posh came close to netting again within 30 seconds of the restart but Carson was alert and did well to clutch Mclean's header from Lee's long throw-in.
But the hosts DID make it 3-2 in the 53rd minute.
Youssouf Mulumbu gifted Posh possession in midfield and Boyd broke swiftly down the left before swinging in a low cross which Mclean slid home at the near post.
Di Matteo made a change a minute later as Koren came on for Wood.
Albion still possessed a real threat and Moore managed to find space in the area to get away a left-footed shot which Lewis held low to his left.
Dorrans' left-wing corner dropped to Koren at the back post on the hour mark and he was unlucky to see his low drive from a tight angle saved on the line by Lewis.
Morgan almost shot his team in the foot in the 63rd minute when he gifted the ball to Moore.
The Baggies' No.16 tried to round Lewis on the edge of the area but the Posh keeper got a touch with his hand and managed to kick the ball away to safety before Dorrans could bear down on the open goal.
Winger Jerome Thomas was handed his Baggies debut in the 67th minute as he came on for Cech.
Albion, who were defending stoutly, regained their composure and Thomas went close to bagging a dream goal three minutes from time when he burst into the box but saw Lewis save at his feet.
Sub Martin almost snatched a point for Peterborough in the fourth minute of stoppage time when he pounced on Joe Mattock's misjudged jump and sent in a swerving 20-yard volley that Carson just managed to hang onto at the second attempt.
Dorrans' competitive nature got the better of him a minute later when his late lunge on Lee earned him a second yellow card of the night - and his marching orders.
PETERBROUGH (4-4-2): Lewis; Lee, Morgan (Rowe 77), Zakuani, Williams (Martin 15); Batt, Diagouraga (Frecklington 63), Coutts, Boyd; Mclean, Mackail-Smith. Subs not used: McKeown (gk), Keates, Pearce, Green.
ALBION (4-4-2): Carson; Cummings, Martis, Olsson, Mattock; Brunt, Dorrans, Mulumbu, Cech (Thomas 67); Moore (Beattie 87), Wood (Koren 54). Subs not used: Kiely (gk), Barnett, Reid, Cox.
GOALS: PETERBOROUGH - Mackail-Smith (45+1), Mclean (53). ALBION - Moore (3, 34), Brunt (41).
BOOKINGS: PETERBOROUGH - Batt (foul 16), Coutts (foul 26). ALBION - Dorrans (foul 50), Martis (throwing ball away 78).
SENDING-OFF: ALBION - Dorrans (foul 90+5 - second booking).
REFEREE: G Horwood (Bedfordshire).
ATTENDANCE: 8,752 (Albion fans 1,470).
ALBION STARMAN: Luke Moore.