First-to-File Rule Law and Legal Definition

First to file rule is a principle of civil procedure that provides that when two suits are brought by the same parties, regarding the same issues, in two courts of proper jurisdiction, the court that first acquires jurisdiction usually retains the suit, to the exclusion of the other court. However, if the first-filed suit is brought merely in anticipation of the true plaintiff's suit and amounts to an improper attempt at forum-shopping then second filed suit becomes effective.

First to file rule also refers to the doctrine allowing a party to a previously filed lawsuit to enjoin another from pursuing a later-filed action.