Emerson Palmieri, a former Chelsea player, scored a goal against his old team to give West Ham a point and put pressure on Blues manager Graham Potter.
Despite taking the lead, Chelsea's new-look £200 million forward line struggled once more, and the team has now won just one of its last seven Premier League games.
Joao Felix scored the opening goal for Chelsea against Benfica on a volley after having an earlier attempt disallowed for offside. Enzo Fernandez, the £107 million British transfer record signing from Benfica, provided a lovely assist.
Kai Havertz thought he had doubled the lead, but Chelsea's celebrations were stopped by a second offside flag before Emerson, a player from Italy, equalized for West Ham with his first Premier League goal.
The 28-year-old, who played for Stamford Bridge for four years, finished at the far post after Jarrod Bowen flicked on Vladimir Coufal's cross.
The better opportunities were created by Chelsea, who have lost 14 points in their last seven Premier League games, but they were unable to convert them.
On his first start for the team since joining for £30.7 million from PSV Eindhoven, Noni Madueke showed plenty of energy. The 20-year-old forced Lukasz Fabianski into a fine save before the West Ham goalkeeper turned away a free-kick from Felix.
West Ham thought they had won it in a dramatic finish when substitute Tomas Soucek scored from close range, but it was disallowed for offside – the third goal of the game to be overturned.
Soucek appeared to handle Conor Gallagher's shot inside his own box in the 89th minute, leading to Potter and Chelsea making strong appeals for a penalty, which were turned down.
Chelsea's stuttering under Potter has continued.
Chelsea will resume their Champions League campaign against Borussia Dortmund on Wednesday, despite having already been eliminated from the FA Cup and the Carabao Cup.
Despite their significant spending over the last two transfer windows, they have only collected seven of the last 21 available points and are far behind in the race to finish in the top four.
Additionally, they haven't experienced a league victory on the road since October, a stretch of seven games that included defeats at Potter's old team Brighton and Hove Albion, Newcastle United, and Fulham.
At West Ham, Chelsea's starting lineup featured seven players under the age of 24, including Madueke, Felix, Mudryk, and Havertz.
The young forward line caused problems in the first half, which were encouraging signs, but Chelsea was disappointing in the second half and has now scored just 23 goals in 22 league games.
Under Potter, who has won just five of his 16 games in the top division since taking over in September, they have continued to sputter.
Paqueta strikes West Ham.
The last time these two teams faced off, at Stamford Bridge in September of last year, Hammers manager David Moyes was incensed by a "scandalous" decision to disallow a 90th-minute West Ham equalizer.
The Hammers once again had a late goal disallowed due to Soucek's finish, but this time there were fewer complaints from the bench because replays clearly showed that it was offside.
Instead, Chelsea felt wronged at the end after Soucek's handball appeals for a penalty were denied.
West Ham is showing signs of resiliency near the bottom of the table after a protracted season of adversity.
On February 19, they travel to Tottenham for another London derby, coming off a four-match winning streak after following up a 1-1 draw at Newcastle with another valiant effort.
Lucas Paqueta, a midfielder for Brazil, may or may not play against Tottenham.
One of West Ham's better players this season, the 25-year-old left the game early due to what appears to be a shoulder injury. Moyes will be hoping it is not too serious as his team attempts to escape trouble.