Rovanpera, who was rallying competitively in Latvia at the age of 12, led from the start in Riga and dominated on the Baltic country's fast gravel roads to win the season's eighth round.
The double world champion beat French teammate and eight-time titleholder Sebastien Ogier by 39.2 seconds. Both are competing this season only part-time.
Estonian Ott Tanak, who was world champion in 2019, finished third for Hyundai and a further 25.3 seconds adrift after overtaking local hero Martins Sesks on the final stage.
Sesks, in only his second WRC rally, dropped to seventh after his M-Sport Ford Puma developed a transmission problem.
"It's been a great weekend," said Rovanpera. "It's cool to have the first win here. Latvia is an important place for me."
Belgian Thierry Neuville finished eighth and retained the overall championship lead for Hyundai, eight points clear of Tanak who moved five ahead of Toyota's Elfyn Evans.
The next round is in Finland from August 1st to 4th.