This paper introduces a least-action formulation developed within the Theory of Imbalance ofEnergy (TIE), in which dynamical degrees of freedom associated with interaction are representedexplicitly rather than being absorbed into instantaneous force laws. The variational principleitself is unchanged. Equilibrium is treated as a constraint on admissible configurations, whileimbalance is introduced as a dynamical contribution to the action that stores and redistributesenergy over finite timescales.The resulting structure yields coupled equations of motion for configuration variables and imbalance variables. In the limit where imbalance redistributes rapidly compared to configurationevolution, the formalism reduces to standard classical mechanics with instantaneous interaction.No applications are presented in this paper, and no empirical claims are made. The purposeof this work is to establish a mathematically consistent, deterministic, and causally well-posedformal foundation for subsequent analyses within the TIE framework.