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Poly(ethylene oxide) (PEO) was the first lithium‐ion conducting polymer developed 50 years ago and is still the most popular electrolyte matrix for solid‐state lithium metal batteries. While many studies focus on increasing PEO ionic conductivity through doping with Li salts, little work has addressed using PEO and Li directly to generate Li+‐conducting species in situ. Reaction between PEO and Li leads to ionic conductivity largely from Li+, in contrast to the case of added salts where the anion contribution dominates. Herein, electrochemical impedance spectroscopy shows the ionic conductivity of PEO thin films increases up to three orders of magnitude (from 10−7 to 10−4 S cm−1) when contacted with Li at elevated temperature. This is due to the reduction of ether bonds, which produces lithium alkoxides that are responsible for Li+ transport. Density functional theory analysis confirms this mechanism as thermodynamically favorable. X‐ray photoelectron spectroscopy also shows the presence of organolithium species and Li2O, which are responsible for propagating reactions with PEO and forming an electronically insulating layer at the PEO–Li interface that halts further reaction, respectively. The underlying mechanisms of Li–polymer electrolyte reactions is clarified and new pathways for in situ Li+ doping of polymer electrolytes is presented.