According to one embodiment, a lithium ion secondary battery is provided. The lithium ion secondary battery includes a negative electrode containing a negative electrode active material-containing layer, a positive electrode, and an electrolyte containing Li ions and Na ions. The negative electrode active material-containing layer contains a Na-containing titanium composite oxide. A ratio (WE/WA) of an Na amount WE (g/g) in the electrolyte to an Na amount WA (g/g) in the negative electrode active material-containing layer satisfies Formula (1) below: 1×10−1≤WE/WA≤1×105  (1).