Mont. Code Ann. § 42-1-106
Who may adopt
42-1-106 . Who may adopt. The following individuals who otherwise meet the requirements of this title are eligible to adopt a child:
(1) a husband and wife jointly or either the husband or wife if the other spouse is a parent of the child;
(2) an unmarried individual who is at least 18 years of age; or
(3) a married individual at least 18 years of age who is legally separated from the other spouse or whose spouse has judicially been declared incompetent.