*

We show that CuAl₂O₄ spinel can host a spin-orbital-entangled J_eff=1/2 state. During the long history of the 3d transition metal study, spin-orbit coupling has never been a dominating energy scale. Here we propose on the CuAl₂O₄ as the first example of a J_eff=1/2 Mott insulator in 3d transition metal compounds. From single crystal X-ray diffraction data, our CuAl₂O₄ is confirmed to have cubic structure without Jahn-Teller distortion and it is consistent with first-principles total energy calculations. Density functional theory combined with dynamical mean field theory calculations reveal that the J_eff=1/2 state survives the competition with the orbital-momentum-quenched S=1/2 state with the help of strong electron correlation.