Enacted effective October 1, 2011 · Last verified June 26, 2026
In one sentenceRule 15 governs changing a pleading — one free amendment within 21 days, leave “freely given” afterward, and relation back so an amendment can share the original filing’s date for limitations purposes.
(1)Amending as a Matter of Course. A party may amend its pleading once as a matter of course within:
(A)21 days after serving it; or
(B)if the pleading is one to which a responsive pleading is required, 21 days after service of a responsive pleading or 21 days after service of a motion under Rule 12(b), (e), or (f), whichever is earlier.
(2)Other Amendments. In all other cases, a party may amend its pleading only with the opposing party’s written consent or the court’s leave. The court should freely give leave when justice so requires.
(3)Time to Respond. Unless the court orders otherwise, any required response to an amended pleading must be made within the time remaining to respond to the original pleading or within 14 days after service of the amended pleading, whichever is later.
(1)Based on an Objection at Trial. If, at trial, a party objects that evidence is not within the issues raised in the pleadings, the court may permit the pleadings to be amended. The court should freely permit an amendment when doing so will aid in presenting the merits and the objecting party fails to satisfy the court that the evidence would prejudice that party’s action or defense on the merits. The court may grant a continuance to enable the objecting party to meet the evidence.
(2)For Issues Tried by Consent. When an issue not raised by the pleadings is tried by the parties’ express or implied consent, it must be treated in all respects as if raised in the pleadings. A party may move -- at any time, even after judgment -- to amend the pleadings to conform them to the evidence and to raise an unpleaded issue. But failure to amend does not affect the result of the trial of that issue.
(1)When an Amendment Relates Back. An amendment to a pleading relates back to the date of the original pleading when:
(A)the law that provides the applicable statute of limitations allows relation back;
(B)the amendment asserts a claim or defense that arose out of the conduct, transaction, or occurrence set out -- or attempted to be set out -- in the original pleading; or
(C)the amendment changes the party or the naming of the party against whom a claim is asserted, if Rule 15(c)(1)(B) is satisfied and if, within the period provided by Rule 4(t) for serving the summons and complaint, the party to be brought in by amendment:
(i)received such notice of the action that it will not be prejudiced in defending on the merits; and
(ii)knew or should have known that the action would have been brought against it, but for a mistake concerning the proper party’s identity.
(2)Notice to the State of Montana and Other Public Bodies. When the State of Montana, local government, or a state or local officer or agency is added as a defendant by amendment, the notice requirements of Rule 15(c)(1)(C)(i) and (ii) are satisfied if, during the stated period, process was served as provided by Rule 4(k) and (l).
(d)Supplemental Pleadings. On motion and reasonable notice, the court may, on just terms, permit a party to serve a supplemental pleading setting out any transaction, occurrence, or event that happened after the date of the pleading to be supplemented. The court may permit supplementation even though the original pleading is defective in stating a claim or defense. The court may order that the opposing party plead to the supplemental pleading within a specified time.
Plain-English Summary
Cases evolve, and Rule 15 lets pleadings keep up. A party may amend once as a matter of course within 21 days (measured from service, or from a responsive pleading or a Rule 12 motion). After that, amendment requires the opposing party’s consent or the court’s leave — and the rule directs courts to “freely give leave when justice so requires.” Pleadings can also be amended during and after trial, including to conform to issues the parties actually tried by consent.
The most consequential piece is relation back. When an amendment arises out of the same conduct, transaction, or occurrence as the original pleading, it is treated as filed on the original date — which can save a claim that would otherwise be barred by the statute of limitations. Special rules govern relation back when an amendment changes a party (tied to the Rule 4(t) service window) or adds the State or a local government. Subdivision (d) allows supplemental pleadings for events that happen after the original pleading.
Frequently Asked Questions
Can I amend my complaint in Montana?
Yes. You may amend once as a matter of course within 21 days; after that, you need the opposing party’s written consent or the court’s leave, which courts freely give when justice requires.
What does “relation back” mean?
It means an amendment is treated as if it were filed on the date of the original pleading, so a claim or party added by amendment can still be timely under the statute of limitations if it arises from the same conduct or transaction.
What is required to add or correct a party by amendment?
Within the Rule 4(t) period for serving the summons and complaint, the new party must have received notice of the action so it is not prejudiced, and must have known the suit would have been brought against it but for a mistake about identity.
Source & verification. Reproduced verbatim from the Montana Code Annotated as
published by the State Law Library of Montana and the Montana Legislature. This rule has not been amended since its adoption.
Adopted by the Supreme Court of Montana (AF 07-0157). Last verified June 26, 2026. ·
Official text
Also known as:amendamended pleadingrelation backleave to amendsupplemental pleading