This work demonstrates that structural defects in amorphous metal oxide electrodes can serve as a reversible Li+ storage site for lithium secondary batteries. For instance, molybdenum dioxide electrode in amorphous form (a-MoO2) exhibits an unexpectedly high Li+ storage capacity (up to four Li per MoO2 unit), which is larger by a factor of four than that for the crystalline counterpart. The conversion-type lithiation is discarded for this electrode from the absence of Mo metal and lithium oxide (Li2O) in the lithiated a-MoO2 electrode and the retention of local structural framework. The sloping voltage profile in a wide potential range suggests that Li+ ions are inserted into the structural defects that are electrochemically nonequivalent. This electrode also shows an excellent cycle stability and rate capability. The latter feature is seemingly due to a rather opened Li+ diffusion pathway provided by the structural defects. A high Li+ mobility is confirmed from nuclear magnetic resonance study.