Adam Clayton scored his first goal for Leeds in stoppage time to deny West Ham a third consecutive league win.
The Hammers looked to be heading for their best start to a season in 12 years after Patrick Kisnorbo scored an own goal.
But Clayton was first to react in the dying minutes after Jonny Howson struck the bar, driving in the equaliser.
Carlton Cole had given the Hammers the lead, but that was cancelled out by Ross McCormack.
It was an absorbing encounter between two sides with Premier League aspirations.
Scott Parker, the subject of transfer speculation all summer, was the best player on the pitch again, and Hammers manager Sam Allardyce will be praying the midfielder will still be at the Boleyn Ground on 1 September.
Leeds had kept just one clean sheet in their last 14 games, and that defensive frailty was exposed on six minutes when Cole wrestled his way on to Matt Taylor's corner, and flicked the ball past keeper Andy Lonergan.
The Hammers continued to pressurise, and Lonergan had to be alert to deny James Tomkins and Kevin Nolan.
The game turned though when Leeds striker Robert Snodgrass hit the woodwork with a superb shot. That seemed to energise the visitors, and they had the chance to level the scores on 26 minutes from the penalty spot after the ball struck Taylor's outstretched arm from a corner.
But Max Gradel, who scored from the spot last weekend, squandered the chance, dragging his shot wide of Robert Green's post.
The half ended with another penalty shout, but Cole was denied by referee Michael Oliver following Aidan White's sliding challenge.
Leeds deservedly equalised in the 59th minute when Andy Keogh and Snodgrass combined well for the latter to cross low into the path of McCormack, who swept the ball past Green.
But West Ham hit back three minutes later when substitute Julien Faubert crossed from the right and Kisnorbo, in attempting to clear the ball, drove it into his own net.
Winston Reid should have tied up the points for the Hammers but was denied at point-blank range from Lonergan, before Parker, watched by England manager Fabio Capello, shot against a post.
And they were made to pay on the stroke of full-time when Howson's shot came back off the crossbar and Clayton buried the rebound to claim a point.