Mont. Code Ann. § 42-4-404
Jurisdiction and venue
42-4-404 . Jurisdiction and venue. (1) The court has jurisdiction over a proceeding for the adoption of an adult or emancipated minor if the petitioner has lived in Montana for at least 90 days immediately preceding the filing of a petition for adoption.
(2) A petition for adoption must be filed in the court in the county in which a petitioner lives.