Carrier frequency offset (CFO) is an important problem in multi-input multi-output orthogonal frequency-division multiplexing (MIMO-OFDM) systems. This letter proposes a blind CFO estimator for MIMO-OFDM systems with constant modulus constellations in the frequency-selective fading channels. A closed-form CFO estimate is also provided, which greatly reduces the computational complexity of the proposed method. Analysis and simulation results demonstrate that the proposed estimator is very effective for MIMO-OFDM systems.