Gianfranco Zola returned to Stamford Bridge and saw his side snatch a vital point which kept Chelsea off the top of the Barclays Premier League.
Zola's struggling side had stunned Chelsea with a 33rd-minute opener from Craig Bellamy but Nicolas Anelka rescued the Blues with his 16th goal of the season five minutes after the break.
It was Chelsea's fourth domestic home game without a win and their failure to take all three points cost them the chance to overtake Liverpool at the top of the table.
Chelsea fans had given coach Zola and assistant Steve Clarke a rapturous welcome as they emerged from the tunnel to take their places in the visiting dugout.
But West Ham soon found themselves under pressure from a home side determined to build on its midweek Champions League victory over CFR Cluj.
In the sixth minute a superb combination between former West Ham players Joe Cole and Frank Lampard almost provided them with an opener.
Cole looked set to score when Lampard's precise pass found him bearing down on the edge of the six-yard box, but his effort was deflected to safety by Scott Parker.
Chelsea started the game at a frantic tempo and after Jack Collison had fouled Ashley Cole on the left flank, Lampard's 16th minute free-kick was headed wide by Alex.
West Ham did exceptionally well to weather the early storm and in the 20th minute John Mikel Obi was booked for bringing down Carlton Cole in full flight.
It gave Zola's side the chance to test Petr Cech with a free-kick some 30 yards out, but Mark Noble elected to chip the ball into the area and Michael Ballack headed clear.
Chelsea's woeful home record took a turn for the worse in the 33rd minute when Zola's struggling side went ahead against the run of play and in controversial circumstances.
Noble looked to have used his arm before crossing the ball for Bellamy to fire the visitors in front from 12 yards.
Ballack was incensed by Mike Riley's decision to allow the goal to stand and was shown a yellow card for protesting.
West Ham, their confidence clearly boosted, enjoyed their longest spell of possession as the interval approached.
In the 45th minute, Anelka finally tested Robert Green with an 18-yard shot that was dealt with comfortably by the West Ham goalkeeper.
Chelsea were booed off at half-time and TV replays showed that Noble had used his shoulder to control the ball instead of his arm in the build-up to West Ham's goal.
Scolari, clearly frustrated with Chelsea's lack of a cutting edge, replaced Ballack with Didier Drogba at the interval.
Drogba hardly had time to get into the game before top scorer Anelka supplied the equalizer in the 50th minute.
For once Chelsea's neat approach play paid off with Mikel and Drogba combining before Lampard chipped the ball neatly into the path of Anelka, who coolly slotted the ball underneath the advancing Green for his 16th goal of the season.
Chelsea were now on top and when Matthew Upson failed to clear a cross, Lampard sent the rebound into Green's midriff from the edge of the penalty area.
West Ham, for the first time in the game, had little option but to defend ever deeper.
The visitors were happy to soak up the pressure on the edge of their own penalty area but that only invited Chelsea to try their luck from distance.
In the 64th minute Lampard let fly from 25 yards and his dipping shot had to be held at the second attempt by Green.
Three minutes later Green pulled off a magnificent save to deny Drogba after he had volleyed Lampard's corner towards the near post.
Salomon Kalou replaced the lackluster Joe Cole in the 73rd minute and his first contribution was to fire a 20-yard shot straight at Green.
West Ham escaped in the 78th minute when Green punched a Lampard corner towards his own goal but it was headed off the line by Parker.
However, Green was the hero seconds later when he flung himself across goal to keep out a header from Alex.
Referee Riley then failed to award Chelsea a penalty when Lampard was tripped by Lucas Neill but it needed a superb save from Petr Cech at the other end to deny Carlton Cole an injury-time winner.
