The Toyota driver benefitted from the defection on Saturday of Sebastien Ogier (39) to wrap up his third win of the year.
The Toyota driver extended his lead at the top of the 2023 standings over teammate Elfyn Evans (34) to 33 points with three rallies remaining.
Evans took second in Greece, over a minute and a half adrift, with Hyundai's Dani Sordo (40) in third.
Evans clocked the fastest time in the first of Sunday's closing three stages, with Sordo claiming the honours in the next and Rovanpera taking the 15th and final Power stage.
Rovanpera's fortunes changed dramatically on Saturday after eight-time world champion Ogier was forced out on the day's last stage when his Toyota suffered a broken suspension as it clattered into a rock.
That allowed teammate Rovanpera, who had been more than 12 seconds in arrears at the time, to finish the day two minutes clear.
The WRC season moves on to Chile at the end of the month, then the Central European Rally, a new addition to the calendar, in Austria, Czech Republic and Germany, with Japan hosting the 13th and final round in November.
