Tortious interference with contract—also known as intentional interference with contractual relations or business expectancy—is a civil claim or cause of action based on interference with a contract or a prospective contract that is about to be completed—by a person or entity who is not a party to the contract (third party).

A claim for tortious interference is based on the idea that the third party encouraged or induced one of the parties to the contract to breach the contract, causing damages to the nonbreaching party, who may sue the third party to recover those damages or losses. In some states there is a requirement that the interference be done maliciously or without justification.

Laws regarding claims for tortious interference with contract vary from state to state. Some states have broadened the protections against interference beyond situations where there is an existing contract and recognize claims for interference with prospective economic advantage or business relations.

But whether there is an existing contract or not, some instances of interference will not create legal liability and will be recognized as legitimate competitive activity, for example.

In Montana, tortious interference with a contract or business expectancy is recognized as a civil cause of action. This occurs when a third party, who is not part of an existing contract, intentionally interferes with that contract or a prospective business relationship, causing harm to one of the parties involved. The third party must have induced or caused one of the contracting parties to breach the contract, resulting in damages. Montana law requires that the interference be intentional and without justification. The state acknowledges the delicate balance between unlawful interference and legitimate competitive behavior. Therefore, not all interferences are actionable—some are considered permissible as part of fair competition. To establish a claim for tortious interference in Montana, the plaintiff typically needs to prove the existence of a valid contract or business expectancy, the defendant's knowledge of the contract or expectancy, intentional inducement to breach or disrupt the contractual relationship or expectancy, and damages resulting from the interference.