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Dendrite penetration in ceramic lithium conductors severely constrains the development of solid‐state batteries (SSBs) while its nanoscale origin remains unelucidated. An in situ nanoscopic electrochemical characterization technique is developed based on conductive‐atomic force microscopy (c‐AFM) to reveal the local dendrite growth kinetics. Using Li7La3Zr2O12 (LLZO) as a model system, significant local inhomogeneity is observed with a hundredfold decrease in the dendrite triggering bias at grain boundaries compared with that at grain interiors. The origin of the local weakening is assigned to the nanoscale variation of elastic modulus and lithium flux detouring. An ionic‐conductive polymeric homogenizing layer is designed which achieves a high critical current density of 1.8 mA cm–2 and a low interfacial resistance of 14 Ω cm2. Practical SSBs based on LiFePO4 cathodes can be stably cycled over 300 times. Beyond this, highly reversible electrochemical dendrite healing in LLZO is discovered using the c‐AFM electrode, based on which a model memristor with a high on/off ratio of ≈105 is demonstrated for >200 cycles. This work not only provides a novel tool to investigate and design interfaces in SSBs but also offers opportunities for solid electrolytes beyond energy applications. [ABSTRACT FROM AUTHOR]