Arla's subsidiary in Finland will merge with the company to produce the Arla Ingman group, and the deal gives Arla three years to buy up the remaining shares in Ingman.
The deal is another example of larger European dairy firms increasing their reach in an attempt to protect themselves from pressure from costs, retailers and lower market support.
Hans-åke Hammarström, Arla's head of Nordic markets, said: "The international consolidation of the dairy industry is steadily continuing and new types of partnership are needed in order to meet consumers' and the retail trade's demand for broader ranges and greater choice."
Arla said the deal showed a clear, long-term commitment to Finnish milk and would therefore bring more stability to milk producers.
Arla Ingman, if approved by EU competition authorities, will have annual sales of around €300m and employ around 500 people.