Denis Stracqualursi's first Premier League goal in the 71st minute sealed a thoroughly deserved win for mid-table Everton as Andre Villas-Boas's side dropped out of the top four.
Dejected Chelsea are now without a win in four league games - a damaging run which threatens their hopes of securing qualification for next season's Champions League.
Manager Villas-Boas could take few positives from a poor performance by his team.
His first away defeat in the league since October 23 could not have come at a worse time after pictures of Chelsea owner Roman Abramovich observing the team training added to speculation over Villas-Boas's future at Stamford Bridge.
Everton are now unbeaten in five with this victory coming hot on the heels of a 1-0 home win over leaders Manchester City.
Chelsea conceded three times in 26 second-half minutes to blow a 3-0 lead against Manchester United in their previous match and they found themselves behind after just five minutes on Merseyside.
Pienaar, making his second home debut after signing on loan from Tottenham last month, scored nine top-flight goals in 104 outings during his first spell at Goodison.
Yet the 29-year-old was on target in just his second appearance since re-joining the club he left 13 months ago.
It was a poor goal to concede from Chelsea's point of view as a Jose Boswinga throw ended up with Tim Cahill challenging Frank Lampard.
The ball spun into the path of Pienaar in the Chelsea area, the former Ajax player controlling the ball with his chest before firing past Petr Cech into the roof of the net.
Pienaar marked the goal with a baby-rocking celebration after Australian international Cahill became a dad again two days before the game.
It was the perfect start for Everton, who had won only one of their previous six league games.
Chelsea, who had Lampard back from a calf injury and Ashley Cole returning from suspension, could not afford any further slips after drawing six of the previous nine games to leave their place in the top four under threat.
But they did not test Tim Howard enough as the Everton keeper lived a charmed life in the first half.
United States international Howard, thanks to his fluke against Bolton on January 4, has scored more league goals than Chelsea striker Fernando Torres in 2012.
Howard had to be alert to deny Torres his first top-flight goal since September 24 after the Spanish striker had out-jumped Johnny Heitinga shortly before half-time.
But Chelsea's frustration bubbled to the surface when former Liverpool midfielder Raul Meireles was booked for kicking Stracqualursi in the chest while trying to challenge the Everton forward.
Torres, who has more bookings than goals for Chelsea, was also cautioned for a cynical second-half challenge on Pienaar.
Chelsea were just as lacklustre in the second half as they were in the opening 45 minutes.
Indeed, it was Everton who looked more likely to score with Argentina's Stracqualursi proving a real threat after the break.
And the Argentinian, on loan from Tigres, wrapped up the points with his first league goal since moving to Goodison in August following a fine move by the home side.
It started with Phil Neville's strong challenge on Cole which allowed United States international Landon Donovan to release Stracqualursi who beat Cech despite the Chelsea keeper getting one hand on the ball.
Source: AFP
Source: AFP