Fully understanding the mechanism of lithium metal deposition is critical for the development of rechargeable lithium battery anodes. The heterogeneous electron transfer kinetics are an important aspect of lithium electrodeposition, but they have been difficult to measure and understand. Here, we use transient voltammetry with ultramicroelectrodes to explicitly investigate the electron transfer kinetics of lithium electrodeposition. The results deviate from the Butler–Volmer model of electrode kinetics; instead, a Marcus model accurately describes the electron transfer. Measuring the kinetics in a series of electrolytes shows the mechanism of lithium deposition under electron transfer control is consistent with the general framework of Marcus theory. Comparison of the transient voltammetry results to electrochemical impedance spectra provides a strategy for understanding how the interplay of the electron transfer and mass transport resistances affect the morphology of lithium.