Dysregulation of Tau in Alzheimer disease affects multiple cellular compartments and functions. Tau interacts directly with and translocates through the plasma membrane, a potential site of Tau fibril and membrane pore formation, but its physiological state and behavior in this compartment are unknown. Using quantitative single-molecule imaging in live cells, we observed that Tau exhibits both confined and free diffusion near the plasma membrane. Preventing Tau/microtubule interactions either pharmacologically or biochemically increased Tau mobility; however, the heterogeneous mobility pattern persisted. Tau formed transient hot spots of ~70 nm in radius, displaying stationary and mobile phases. Compared to the ~40 ms dwell time of Tau on microtubules, the hot spots were long-lasting (tens of seconds), and resistant to microtubule perturbations and cholesterol depletion. We suggest that the mobile pool acts as a reservoir supplying Tau to functional sites, with the hot spots serving as assembly sites for the physiological and pathological functions of Tau.