Pearce, who has missed large parts of recent seasons through injurires and hadn't played since the visit to Aston Villa on March 2, crept unmarked to thunder a left foot shot past Kasey Keller from 22 yards after Steve Lomas' right wing cross was only half cleared by Simon Davies.
It was Pearce's fifth league goal for the club and his first in almost 18 months - since scoring in a 4-1 home win over Manchester City on November 11, 2000.
Although Hammers' late equaliser stunned the 36,083 White Hart Lane crowd, Spurs having dominated for long periods of the second half, a point each was a fair outcome because Hammers had created the best chances of a scrappy first half bathed in the north London sunshine.
With Paolo Di Canio (knee) and Joe Cole (thigh strain) both sidelined, manager Glenn Roeder shuffled the pack, pairing Jermain Defoe up front with Fredi Kanoute, who were supported from behind by the central Trevor Sinclair. Lomas and Michael Carrick did well to control midfield in the first half, although Vladimir Labant had a torrid time on the left and was regularly beaten by Davies, who supplied a string of dangerous crosses for Teddy Sheringham and Sergei Rebrov.
Christian Dailly, who made a fine, last gasp tackle to deny Rebrov in the 29th minute, Pearce and Tomas Repka looked assured in a back three that saw Labant and Sebastien Schemmel asked to cover plenty of ground from the wing-back positions.
Despite suffering from a bout of tonsillitis, Defoe looked sharp and caused Spurs' back three of Chris Perry, Anthony Gardner and Ben Thatcher plenty of headaches. He brought a low, diving save from Keller and, in the 22nd minute, set up Hammers' best chance. Receiving Sinclair's headed flick, Defoe raced to the left byline and delivered a sharp cross to the near post area, but Kanoute directed his shot wide of the post from close range.
Both teams, renowned for their neat passing play, struggled to find much cohesion and too often surrendered possession easily. Spurs stepped up a gear at the start of the second half, sending on Steffen Iversen for Rebrov, and his aerial threat immediately made an impact.
After Kanoute twisted and turned the Tottenham defence to fire a shot that had Keller scrambling across goal, Spurs' pressure paid off with the opening goal on 52 minutes.
Davies set Iversen clear through the middle and although David James did well to block his right-foot shotn on his six-yard line, the ball rebounded kindly to Sheringham, who tucked it away from penalty spot range.
It was important to stem the supply from wide areas, so there was no surprise when Roeder replaced the struggling Labant with the more defensively minded Wiunterburn, in the week in which it was revealed the 38-year-old veteran would be given a free transfer at the end of this season.
Tim Sherwood, arriving unmarked in the penalty area, could have wrapped it up for Spurs after 74 minutes, but his chest control and volley was brilliantly saved by the legs of James.
Four minutes later and Sinclair's dipping volley from 25 yards represented only the second goal attempt by Hammers after the interval. Carrick then angled a free-kick that drifted agonisingly wide of the far post and it seemed as though West Ham's three-match winning run was about to end in defeat.
But then Ian Pearce came from nowhere to score one of the goals of the season and, in his case, perhaps a lifetime!
Minute-by-minute:
3 - Rebrov's deep cross is just inches from Poyet's lunge at the far post and the ball runs out for a throw-in.
5 - Lomas collects the ball on the halfway line and evades two challenges before scuffing a low shot straight at Keller.
7 - After Kanoute's surging run is halted by Thatcher, Schemmel's shot is deflected behind for a corner.
9 - Defoe explodes into the game as his clever dummy reaches Kanoute, who returns the ball for the young striker to skip past two defenders and fire in a blistering shot that is well held by Keller.
16 - Schemmel's looped flick into the box causes panic and Defoe's challenge against Perry sees the ball fall for Kanoute, whose stretching header is gathered by Keller.
23 - Defoe once again proves his danger as he collects Sinclair's flick-on and bursts into the box before cutting the ball back along the six-yard box, where Kanoute stabs the ball just wide of the post.
29 - Anderton's slide-rule pass almost releases Rebrov, but Dailly's saving tackle clears the danger.
31 - Carrick dispossesses Anderton in midfield and then advances forward on to his left foot before firing a shot high over the bar.
37 - After Labant's low cross isn't dealt with by the Spurs defence, the ball runs loose to Defoe, who fires in a shot from an acute angle that Keller spills but then grabs at the second attempt.
42 - Kanoute's hard work pays off again as he snatches a loose ball in midfield and surges towards goal, but the Frenchman's shot from 25 yards out is well wide of the target.
45 - Some good build-up play involving Labant and Lomas sees the ball played out to Sinclair, who is allowed to dribble forward unchallenged but fires his shot wide.
Added time: 1 minute.
Half-time: Tottenham Hotspur 0 West Ham United 0.
Second Half:
46 - Substitute for Tottenham Hotspur, Iversen replaces Rebrov.
49 - Hammers start the second half in menacing mood, as Kanoute turns Thatcher twice and unleashes a blistering effort that Keller palms away to safety.
53 - GOAL. Spurs take the lead against the run of play, as Davies picks up a loose ball in midfield and threads a pass through that is missed by Repka and drops into the path of Iversen. The Norwegian striker fires in an effort that is superbly blocked by James, but Sheringham is following up to stroke the ball home. Tottenham Hotspur 1 West Ham United 0.
57 - Davies' low shot is spilled by James, but the England keeper redeems himself with a fantastic block to deny Sheringham a second follow-up goal in the space of four minutes.
59 - Kanoute ghosts behind the Spurs defence to latch on to Lomas' pass and holds off Thatcher before cutting the ball back into the area, but unfortunately no-one is waiting to finish the move off.
60 - Substitution for West Ham, Winterburn replaces Labant.
63 - Sinclair collects a return pass from Kanoute on the left and hangs up a cross that Defoe can only nod wide.
75 - James once again comes to the rescue as Sherwood skips into the box unmarked to meet a Davies cross and blasts a volley that the Hammers keeper blocks with his legs.
78 - Sinclair almost grabs a stunning equaliser with a dipping volley from 30 yards out that is clearly tipped over by Keller, although not according to referee Barry, who awards a goal-kick.
82 - Carrick's free-kick from the left is flicked on by Dailly, but a further touch from a Spurs defender sees the ball evade Lomas by inches at the far post and drift out for a corner.
84 - Substitution for Tottenham, Doherty replaces Ziege.
88 - GOAL! Hammers get the equaliser they deserve, from the most unlikely of sources. Lomas' cross from the right is only half cleared by Davies, and Ian Pearce - on the edge of the box for some strange reason! - smashes an unstoppable left foot shot into the top corner of the net for his first goal of the season. Tottenham Hotspur 1 West Ham United 1.
Added time: 2 minutes.
Full-time: Tottenham Hotspur 1 West Ham United 1.