Chelsea's Champions League hopes stuttered after they suffered a night to forget in Rome.
Christian Panucci took advantage of some unusually slack Chelsea defending to finish off a Cicinho cross.
Mirko Vucinic doubled the lead with a wonder strike and then added a superb solo effort to ensure the victory.
John Terry tapped in a consolation from a cross by Deco, who was then dismissed after being shown a second yellow card for attempting a quick free-kick.
Chelsea went into the game with hopes of securing their place in the knockout stages, but instead they will go to Bordeaux in three weeks' time with the group wide open after this defeat.
It was a result that went completely against the form book, with Chelsea arriving at the Stadio Olimpico having gone top of the Premier League at the weekend and Roma on a run of four Serie A defeats.
The opening period suggested it would go to plan with Roma keeper Doni kept busy by shots from Deco, Florent Malouda and Frank Lampard.
Roma looked like a team short on confidence, with some sloppy passing and a deep-lying defensive line allowing Chelsea to spend much of the early stages camped in their half.
Vucinic gave them a warning of what was to come when he created a chance from nothing by controlling the ball on his chest and volleying over the bar.
Deco and Lampard played a lovely one-two in the build-up to the England midfielder testing Doni's reactions with a 25-yard drive that was headed just inside the post.
However, with Chelsea seemingly in firm control they slipped behind through no fault but their own.
Some careless passing resulted in Deco needlessly fouling Francesco Totti about 35 yards from goal, earning the Chelsea midfielder his first booking.
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Worse was to follow when the free-kick was played to Cicinho down the right and his centre found Panucci, who was left free to glance the ball home.
It was only the second goal Chelsea had conceded in 916 minutes of football and just the second in open play for Roma in six games.
The Italians grew in confidence and Vucinic could have added a second just before half-time when he misjudged a Cicinho cross.
The big Montenegrin got his first goal three minutes after the restart when he received a pass from Matteo Brighi and thumped a long-range shot past Petr Cech.
The 25-year-old capped a fine individual display when he robbed John Obi Mikel in the Chelsea half and headed for goal. He brushed off Mikel's attempts to stop him before sweeping the ball past Cech.
Terry pulled one back when he got in the way of a drilled cross from Deco.
Doni saved the initial effort but was powerless to stop him side-footing the rebound in from close range as Roma's defenders appealed desperately for a combination of handball and offside.
Spanish referee Luis Medina Cantalejo showed Deco a second yellow card for attempting to take a quick free-kick, denting any hope of a late comeback and compounding Chelsea's misery.
They will be hoping it is not a sign of things to come on a ground which will stage the Champions League final this season.
Roma: Doni, Cicinho, Mexes, Juan, Panucci, Perrotta (Taddei 72), De Rossi, Brighi, Pizarro, Totti (Julio Baptista 61), Vucinic (Riise 88).
Subs Not Used: Artur, Loria, Tonetto, Menez.
Booked: Perrotta.
Goals: Panucci 34, Vucinic 48, 58.
Chelsea: Cech, Bosingwa (Kalou 63), Terry, Alex, Bridge, Deco, Mikel, Lampard, Joe Cole (Belletti 46), Anelka, Malouda (Drogba 46).
Subs Not Used: Cudicini, Ivanovic, Di Santo, Ferreira.
Sent Off: Deco (81).
Booked: Deco.
Goals: Terry 75.
Att: 38,425.
Ref: Luis Medina Cantalejo (Spain).
BBC Sport Player Rater man of the match: Roma's Mirko Vucinic (6.96 on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.
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