We have carried out classical molecular dynamics simulations to study the configurational and energetic properties of the Si self-interstitial. We have shown that the Si self-interstitial can appear in four different configurations, characterized by different energetics. Along with the already known tetrahedral, dumbbell, and extended configurations, we have found a highly asymmetric configuration not previously reported in the literature. Using a data analysis technique based on time averages, we have extracted the formation enthalpies and the probability of finding the interstitial in a given configuration, both depending on temperature. By the use of thermodynamic integration techniques we have determined the Gibbs free energy and entropy of formation, and the relative concentration of each interstitial configuration as a function of temperature. We have demonstrated that the change of interstitial configuration is correlated with the diffusion process, and we have identified two different mechanisms for interstitial-mediated self-diffusion. In spite of the microscopic complexity of the interstitial-mediated diffusion process, our results predict a pure Arrhenius behavior with an activation energy of 4.60eV in the temperature interval 900–1685K, in good agreement with experiments. This energy is decomposed in an effective interstitial formation enthalpy of 3.83eV and a migration barrier of 0.77eV, which macroscopically represent the averaged behavior of the different interstitial configurations.