Emerson Palmieri, a former Chelsea player, scored a goal for West Ham against his old team to give the Hammers a point and put Blues manager Graham Potter under more pressure.
Chelsea played with a brand-new £200 million forward line, but despite taking the lead, they were once again frustrated and have now lost all seven of their Premier League games.
Following a wonderful assist from Enzo Fernandez, the £107 million British transfer-record signing from Benfica, Joao Felix of Portugal gave visiting Chelsea the early lead with a volleyed finish. Felix had already had a goal disallowed for offside.
Prior to Emerson's first Premier League goal for West Ham, Kai Havertz thought he had doubled the lead before Chelsea's celebrations were stopped by a second offside flag.
After Jarrod Bowen flicked on Vladimir Coufal's cross, the 28-year-old, who spent four years at Stamford Bridge, finished at the far post.
Chelsea had the better opportunities but were unable to convert them, dropping 14 points in their last seven Premier League games.
Noni Madueke was active in his first game since joining the team for £30.7m from PSV Eindhoven, and the 20-year-old forced Lukasz Fabianski into a fine save before the West Ham goalkeeper stopped a free-kick from Felix.
West Ham thought they had won it in the dying moments when substitute Tomas Soucek scored from close range, but it was overruled for offside – the third goal that had been disallowed in the game.
A strong appeal for a penalty in the 89th minute after Soucek appeared to handle Conor Gallagher's shot inside his own box was denied, adding to Potter and Chelsea's frustration on this extremely frustrating day.
Chelsea keeps faltering under Potter.
Following a poor run of results, Chelsea, who have already been eliminated from the FA Cup and the Carabao Cup, will begin their Champions League campaign against Borussia Dortmund on Wednesday.
Despite their significant spending over the past two transfer windows, they have only taken seven of the last 21 available points and are far behind in the race to finish in the top four.
They have also lost seven straight games, including defeats at Potter's old team Brighton and Hove Albion, Newcastle United, and Fulham, and they haven't won on the road in the league since October.
At West Ham, Chelsea's starting lineup included Madueke, Felix, Mudryk, and Havertz, as well as seven players under the age of 24.
Although the young forward line caused some issues in the first half, Chelsea's performance in the second half was disappointing. They have now scored just 23 goals in 22 league games.
Under Potter, who took over in September but has only won five of his 16 games in the Premier League, they have continued to sputter.
Paqueta delivers a blow to West Ham.
West Ham manager David Moyes was incensed by a "scandalous" decision to disallow a 90th-minute equalizer during the two teams' previous meeting at Stamford Bridge in September.
As Soucek's finish was ruled to be offside, they once again had a late goal chalked off, but this time, there were fewer complaints from the Hammers bench because replays clearly showed it.
Instead, Chelsea felt wronged at the end when their requests for a penalty following Soucek's handball were denied.
West Ham is starting to fight near the bottom of the table after a protracted season of difficulty.
On February 19, after following up a 1-1 draw at Newcastle with another determined effort, they travel to Tottenham for a second London derby. They are currently on a four-match winning streak.
It is unknown if Brazil midfielder Lucas Paqueta will play against Tottenham.
The 25-year-old, who has been one of West Ham's better players this season, was taken off with what appears to be a shoulder injury, and Moyes will be hoping it is not too serious as his team attempts to escape trouble.