Tanak, who left Hyundai for M-Sport at the end of last year, finished 18.7 seconds clear of Hyundai's Irish part-timer Craig Breen (33) for Ford's first win since Sebastien Loeb (48) in Monte Carlo last year - a wait of 385 days.
Hyundai's Thierry Neuville (34) was third with Toyota's reigning world champion Kalle Rovanpera (22) fourth.
Tanak now has 41 points to Rovanpera's 38 with Neuville third on 32.
"It obviously means a lot to me, to come into a new car and deliver to these guys," said Tanak. "It's a big effort for them to fight against such big manufacturers."
Breen took a strategic 10-second penalty before the final stage to lift Neuville, who is contesting the full championship for Hyundai, into second place but the Belgian then lost time and ended up 1.3 seconds behind the Irish driver.
Toyota stayed top of the manufacturers' standings, 14 points clear of Hyundai.
