Entitled Syötäväksi kasvatetut, Miten ruokasi eli elämänsä (Bred to be Eaten: How Your Food Lived its Life), the winner was chosen from a shortlist of five by Janne Virkkunen, long-time editor of the newspaper Helsingin Sanomat.
He praised the author for her fearless investigative journalism and balanced, calm presentation of the facts.
According to Virkkunen, Lappalainen's comprehensive work show in well-written Finnish what kinds of conditions laying hens, broiler chickens, pigs and cows live in.
Last month, Lappalainen's book also won Kanava magazine's book prize.
Elina Lappalainen, who was born in 1984, works for the financial weekly Talouselämä. The prestigious award is worth 30,000 euros.
The Finlandia prize for fiction will be handed out on December 4 by former President Tarja Halonen.