Javier Hernandez's equaliser dented Chelsea's bid for Champions League football and earned West Ham a point in their bid to survive relegation.
Former Manchester United striker Hernandez, on as a substitute, levelled things up 17 minutes from time, cancelling out Cesar Azpilicueta's first half opener.
The draw leaves Chelsea in real danger of missing out on Champions League football for next season, with the champions 10 points behind Tottenham in fourth. West Ham, meanwhile, moved six points clear of the bottom three following a credible draw in a lively London derby.
Chelsea started the match with their Champions League hopes for next season in jeopardy, following a run of five defeats in seven.