Reigning double world champion Rovanpera had led Ogier by 45.8 seconds and seemed destined to become the first Finn to win in Finland since Esapekka Lappi in 2017, but it was not to be.
The 23-year-old clipped a large rock and crashed out less than 2 km from the end of the stage, handing the lead to eight-time world champion Ogier for his 61st rally victory and second in Finland since his first in 2013.
Hyundai's world championship leader Thierry Neuville was second and M-Sport Ford's Adrien Fourmaux finished third.
"It's hard to smile right now," said Ogier. "A win in Finland is always nice but that is not the way we like to have it. Very sorry for Kalle and (co-driver) Jonne (Halttunen). It's a shame for them."
Neuville leads Ogier by 27 points in the drivers' championship, on 168 to the Frenchman's 141 with Hyundai's Ott Tanak dropping to third on 137 and four rounds remaining.
Tanak crashed out on Friday and did not rejoin.
Ogier, like Rovanpera, had intended to do only a part-time season but the Frenchman is now Toyota's main challenger for the title after Elfyn Evans suffered driveshaft problems on Saturday, and he recognised his plans might have to change.
"I have to see but it looks like I have no other option," he said. "It is not my priority any more but we'll see."
Hyundai are 20 points clear of Toyota in the manufacturers' standings.
