After a Romelu Lukaku double put the Baggies 2-0 up, an unbelievable three goals in the final 10 minutes saw the hosts secure a quite astounding and important victory
The result, coupled with QPR's draw with Tottenham earlier in the day, sees the Royals put some breathing space between themselves and the league's bottom-boys.
Former Reading striker Shane Long missed the game with injury but Steve Clarke recalled Lukaku, along with the likes of Jonas Olsson and Ben Foster. Brian McDermott, meanwhile, brought Pavel Pogrebnyak, Danny Guthrie and Jobi McAnuff back into the side and handed new signing Daniel Carrico a start.
It took just over 15 minutes for the visitors to carve out the first chance when James Morrison forced Adam Federici into a fine save after George Thorne's cross from the right, the midfielders both allowed far too much room.
However, the Royals keeper was helpless to deny Lukaku opening the scoring minutes later. The Belgian, on loan from Chelsea, started the move by bringing down a long ball in the middle of the field, he then shifted it on to Morrison, and continued his run into the box, before his team-mate provided a simple cross that left the forward with an even simpler finish.
The remainder of the half saw very little action, but if Jimmy Kebe's touch after Danny Guthrie's searching through ball had have been better, the attacker would have been left with a brilliant chance to equalise.
It was not though, and as it turned out Lukaku may well have doubled the Baggies' lead just seconds after the second half began, but his strike was blocked by a fantastic sliding challenge by Adrian Mariappa. The Belgian did then put the ball in the back of the net again, only for it to be, correctly, ruled out for offside.
Lukaku did, however, finally get his, and West Brom's, second goal of the game with just over 20 minutes remaining. The forward received the ball from James Morrison before running at the Royals defence, who then backed off as he duly slotted the ball past Federici at the keeper's near post.
The Royals launched a late surge when Kebe popped up at the back post to head McCleary's cross into the back of the net with 10 minutes to play, the delivery evading everyone else.
And the unlikely comeback was on when Adam Le Fondre slotted home a late, late penalty with just three minutes left. Olsson brought down Kebe inside the box, and the forward sent Foster the wrong way.
The Madejski Stadium then erupted when, miraculously, Pogrebnyak beat two West Brom defenders to a goal-mouth scramble to put the hosts 3-2 up and secure all three points in extraordinary fashion.