Following the sides' thrilling 3-3 draw in London two weeks ago, the Italy striker's brace was the highlight of another impressive Roma display which allowed them to leapfrog the English champions at the summit of Group C.
El Shaarawy's thunderous first-minute goal was the earliest in the competition this term and, despite Chelsea creating early chances, Antonio Rudiger's defensive indecision allowed the striker to nip in for a neatly-taken second after 36 minutes.
Not for the first time this season, Chelsea again looked uncharacteristically disorganised at the back as Diego Perotti thrashed home a third in the 63rd to move Roma on to eight points, one clear of Chelsea, who slip to second.
The Premier League side's only consolation after their nightmarish Halloween performance was that they were still effectively in control of their destiny after Atletico Madrid's surprise home draw with Qarabag left them with three points.
After the thrilling fare in their first clash at Stamford Bridge, the one-sided nature of the return, which saw Chelsea's biggest defeat under Antonio Conte since a 3-0 loss at Arsenal last season, was a sign of unsettled times for the Italian.
"The second half was a really bad performance," Conte told BT Sport. "They (Roma) showed more will and more desire to win the game and I'm very disappointed with our second half.
"We all have to take responsibility. You must be worried when you lose like this. It means something doesn't work," added the Italian, conceding that Chelsea must work on tactics before their Premier League match with Manchester United on Sunday.
DRAMATIC FARE
The game instantly exploded into the same sort of dramatic fare that had decorated the Stamford Bridge match.
When Aleksandar Kolarov's hoofed ball from the left was nodded back fortuitously by Edin Dzeko in Roma's opening foray, El Shaarawy was on to the loose ball in a flash to send a right-foot shot on the half-volley crashing past Thibaut Courtois.
Chelsea countered brightly with Eden Hazard causing problems for the home defence, Alvaro Morata hooking a shot over from close range and Tiemoue Bakayoko heading just wide.
But when former Roma defender Rudiger had a lapse of concentration, allowing El Shaarawy to pounce on a speculative ball forward from Radja Nainggolan and nudge his effort expertly past Courtois, Chelsea's poise disintegrated.
Most shocking for Conte was how tamely his side capitulated after the break, looking nothing like the team which looked so solid throughout their Premier League title-winning campaign.
Cesc Fabregas, in his 100th Champions League match, typified the malaise by giving the ball away to kick start the move that saw Perotti strike a fierce 25-metre shot past Courtois.
The Argentine should then have made it four but ballooned his shot high and Courtois had to make two fine saves in a minute, including a dazzling stop at point-blank range from substitute Kostas Manolas, to spare Chelsea further indignity.
(Reporting by Ian Chadband; Editing by Toby Davis and Ken Ferris)
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