The Italian maestro received a warm and encouraging embrace from caretaker manager Trevor Brooking as he was summoned from the subs' bench 10 minutes into the second half.
After 70 minutes, everybody in claret and blue wanted to bow down at the feet of the Roman 'god', who calmly and confidently placed Trevor Sinclair's deflected right wing cross into the roof of the net, beyond the reach of his brilliant compatriot Carlo Cudicini.
The win lifted Hammers out of the bottom three and, regardless of whether Bolton Wanderers win at Southampton later this evening, at least this extended run of only one defeat in 10 games ensures that the relegation issue will not be decided until the final day of the season.
And how important will Brooking's decision to recall Di Canio to the squad prove to be after the maverick playmaker had spent more than two months nursing a gastric illness and, maybe, a little wounded pride, too?
Hammers produced a performance that, had it been witnessed in August, would have left even the most cynical Cockney pondering the real prospect of European football at Upton Park next season, not visits from Crewe and Wigan. Make no mistake,West Ham deserved this victory, which completes a Premiership double over the West Londoners, who are still likely to secure Champions' League qualification on Sunday week. If they don't pip Liverpool for fourth place, they will have every reason to curse Di Canio, who has scored three goals and orchestrated two victories against them this season.
Brooking, recovered from the draining experience of his debut managerial success at Manchester City, signalled his attacking intent from the start by picking three strikers in Fredi Kanoute, Jermain Defoe and Les Ferdinand, who recovered in time from his Maine Road knock, as did skipper Joe Cole, who joined Steve Lomas and Sinclair in a tight midfield trio.
Chelsea's back four knew they would be up against it from the opening moments as the Irons created three good scoring chances in the first 12 minutes. Cudicini staked his claim as the Premiership's top keeper with a superb, flying save from Kanoute's flicked header and another world class stop from a Sinclair volley.
But Cudicini would have had no chance had Lomas got more that just the faintest of touches to Rufus Brevett's inswinging corner three minutes before the break. It missed by a whisker.
David James had precious little to concern him at the other end. While Chelsea typically passed the ball well in midfield, where they always outnumbered the home side four-to-three, and occasionally by one more when Mario Melchiot made his striding runs from right-back to join the attack, the Blues lacked a killer finish. Former Hammer Frank Lampard, whose every touch was jeered, almost headed Chelsea in front but Brevett produced another goalline clearance to equal his Maine Road heroics.
It was much the same in the second half, too, as Hammers, urged forward by the full might of their desperate fans, should have taken an early lead. Sinclair turned Melchiot one way and then the other before picking out Kanoute, but the Frenchman's towering far post header flew agonisingly wide of the post.
For a change, the quality of West Ham's crossing was vastly improved to much of what had gone before this season and, on the hour, Sinclair chipped another far post ball to Lomas that he headed powerfully at the Chelsea keeper.
Ferdinand had run his socks off for 55 minutes when Brooking decided it was time for Di Canio to return for the first time since his unhappy exit at West Brom on February 23. Chelsea boss Claudio Ranieri admitted afterwards that he feared the danger that his fellow Italian could cause and he was proved right.
While Chelsea wouldn't have been happy with their inability to deal with Sinclair's cross from the right, both Babayaro and Gallas failing to deal with it before it reached Di Canio's right foot, credit the West Ham No.10 for taking up such an intelligent position and the cool execution of his finish.
Nothing much else happened. Chelsea replaced both Eidur Gudjohnsen and Gianfranco Zola, who were both very well marshalled by the immaculate Tomas Repka and Christian Dailly, although neither of their replacements, Jimmy Floyd Hasselbaink and Carlton Cole, could make an impact either. Even allowing for the apparent and understandable nervousness of teenager Glen Johnson, this was the most cohesive and resilient performance by West Ham's defence all season.
The only late scare for Hammers came deep into time added on, when Frank Lampard threatened to ram the boos back down the throats of his relentless tormentors with a rasping, dipping shot from 25 yards that James did well to turn round the post.
Lampard was not going to steal Di Canio's thunder, though. If this was his last-ever appearance in a claret and blue shirt at Upton Park, then as farewells go, this was pretty spectacular.
As referee Andy D'Urso blew the final whistle, a very emotional Paolo whipped off his shirt, sunk to his kness in prayer for Hammers' hopes and then made that slow, familiar walk to the fans in the Bobby Moore lower, where he handed the coveted No.10 jersey to a beaming fan.
You couldn't make it up.
By Tony McDonald
Minute-by-minute:
2 - An early chance for the Hammers in a bubbling atmosphere at Upton Park, as Sinclair's cross from the left reaches Kanoute, who wildly scuffs his attempted shot, but the ball drops kindly for Defoe, whose low volley is held by Cudicini.
7 - Hammers are flying and after Brevett's corner is cleared back out to him, his second cross is met by Kanoute, whose header is superbly tipped over by Cudicini.
12 - This time it's Ferdinand who goes close, as he strides on to Kanoute's nod-down and volleys a blistering effort just over the bar.
13 - A long ball over the top looks to have released Zola in the box, but James rushes out to bravely punch clear as the Italian shapes to shoot.
19 - Sinclair booked for foul on Gudjohnsen.
25 - Hammers are getting closer, but again Cudicini comes to Chelsea's rescue. Lomas' pass releases Johnson, whose low cross from the right is deflected up and out to the edge of the box, where Sinclair unleashes a fierce volley that the Italian keeper blocks with his legs. Ferdinand can't make amends with the rebound.
27 - The Upton Park old boy Lampard almost silences his critics as he rises unmarked six yards out to meet Le Saux's corner. However, Brevett thankfully clears off the line.
42 - Another glorious chance goes begging, as Brevett's dangerous corner misses out Cudicini and finds the run of Lomas, who somehow directs his point-blank header wide of the target from just a few yards out.
Added time: 1 minute.
Half-time: West Ham United 0 Chelsea 0.
Second Half:
55 - Sinclair does well on the left to turn Melchiot and hang over a cross that is met by Kanoute, whose header flashes just inches past the post.
56 - Substitution for West Ham, Di Canio replaces Ferdinand.
62 - Again Sinclair does well to evade Melchiot and send over a cross that is headed goalwards by Lomas, but tipped over by Cudicini.
71 - GOAL! The script was written and the leading man delivered. Paolo Di Canio, having seemingly bowed out of Upton Park without a whimper, returns in stunning fashion. A neat move involving Kanoute and Cole sees the ball worked out wide to the right where Sinclair sends in a low cross that is deflected out to Di Canio, who fires home from eight yards out. West Ham United 1 Chelsea 0.
72 - Double substitution for Chelsea, Hasselbaink replaces Gudjohnsen and Cole replaces Zola.
73 - Defoe booked for dissent.
76 - Repka booked for foul on Hasselbaink.
77 - Substitution for Chelsea, Zenden replaces Morris.
Added time: 4 minutes.
90 - Lampard almost silences Upton Park with a rasping 30 yard effort that James just manages to claw round the post.
Final Score: West Ham United 1 Chelsea 0.
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