The Toyota driver was one of two Rally1 entries to complete the event's attritional 15 stages without a delay, affording the Finn a comfortable winning margin of 1m31.7s over title rival and team-mate Elfyn Evans.
Rovanpera led the rally after winning Thursday night's asphalt super special in Athens but dropped to third when the rally tackled Greece's notoriously rough gravel roads.
The stages were made even more challenging thanks to torrential rain earlier in the week, courtesy of Storm Daniel, which disrupted preparations and forced organisers to cancel Thursday's shakedown.
Facing the disadvantage of starting first on the road, Rovanpera was able to limit the time loss, helped by wet and muddy conditions on Friday morning. His only scare came when he almost rolled on stage three after hitting a compression caused by the extreme weather.
The Finn headed into Saturday's, much drier and traditional stages, 25.5s adrift of leader Hyundai's Thierry Neuville, who headed Toyota's part-time driver Sebastien Ogier.
Neuville and Ogier were locked in a battle for the victory, with the latter briefly overhauling the Belgian on Saturday morning, before Neuville fought back to open up a 10.9s lead.
Rovanpera brought himself into the victory fight after reeling off four consecutive stage wins, before being thrust into a shock lead on Saturday afternoon when Neuville and Ogier both hit trouble.
Neuville hit a hole on stage nine, causing terminal damage to his i20 N's front right suspension, handing the lead back to Ogier.
However, on stage 12, the final test of the afternoon loop, Ogier picked up a double rear puncture. But the collapse of his left rear suspension moments later after an impact ultimately forced the eight-time world champion into retirement.
Having avoided punctures or mechanical issues, Rovanpera was presented with a sizeable 2m04.4s lead which afforded the championship leader the opportunity to cruise to victory.
Rovanpera boosted his points tally by beating Evans by 2.5s to win the Power Stage, which helped extend his championship lead over Evans to 33 points.
Evans produced an impressive fightback from a mechanical issue on Saturday to finish second. The Welshman suddenly slowed during stage nine with a suspected overheating problem on his GR Yaris that cost him more than a minute, dropping him to fifth.
A roadside repair kept Evans in the fight rising back to third behind Hyundai's Dani Sordo following retirements for Neuville and Ogier. Evans became embroiled in a close fight with Sordo, who managed to avoid any problems to steadily climb the leaderboard.
Sordo was unable to hold off Evans, losing out on second spot by 4.2s.
M-Sport's Ott Tanak recovered from a 3m40s penalty for being 22 minutes late from a Friday tyre fitting zone due to a sudden water pump failure, to finish fourth (+4m28.4s).
Tanak dropped to 32nd after the penalty but climbed back to the same position he held before the mechanical problem occurred.
Tanak's team-mate Pierre-Louis Loubet could only complete Thursday's super special after suffering a terminal water pump issue on Friday morning.
Hyundai's Esapekka Lappi encountered several delays on his way to fifth (+6m22.3s). The Finn damaged his radiator on Friday which required a trip to a restaurant to fetch water to keep his i20 N going.
He was further held up by alternator and driveshaft failures but was able to nurse the car through to the finish.
Toyota's Takamoto Katsuta was the last of the permanent Rally1 drivers to complete the full distance in sixth (7m20.9s). The Japanese lost more than four minutes following a double puncture in stage 11.
In WRC2, Andreas Mikkelsen produced an incredible recovery from three Friday punctures to rise from 12th in class to take victory, finishing seventh overall in the process. Gus Greensmith, Yohan Rossel and Ogier, who rejoined the action on Sunday, completed the top 10.
