A premarital agreement must be in writing and signed by both parties. It is enforceable without consideration.
Mont. Code Ann. § 40-2-604
Formalities
Known as the Uniform Premarital Agreement Act
The act spans §§ 40–40 (10 sections).
En
Official source: Montana Code Annotated (Montana Legislature). Reproduced from public-domain Montana statutes; confirm against the official source for the current text. Not legal advice.