We synthesized crystalline bulk samples of lanthanum cuprate in the metastable T′ phase using cesium hydroxide flux. Its crystal structure was determined as space group I4/mmm, no. 139, a=401.02 pm, c=1252.66 pm. Muon spin rotation reveals a gradual slowing down of magnetic fluctuations below TN1≈200 K, and static magnetic order below TN2=115 K, in sharp contrast to La2CuO4 in the T structure where TN1≈TN2≈300 K. Our result shows that the strikingly different magnetic behavior of the two parent compounds has its origin in the two crystal structure modifications. In addition, we find that T′-La2CuO4 has strongly reduced magnetic interactions compared to the other T′ materials Nd2CuO4 and Pr2CuO4, where Nd3+ and Pr3+ are magnetic ions in contrast to the nonmagnetic La3+.