We describe a new precoder based on optimization of the minimum Euclidean distance dmin between signal points at the receiver side and for use in multiple-input multiple-output (MIMO) spatial multiplexing systems. Assuming that channel state information (CSI) can be made available at the transmitter, the three steps noise whitening, channel diagonalization and dimension reduction, currently used in investigations on MIMO systems, are performed. Thanks to this representation, an optimal dmin precoder is derived in the case of two different transmitted data streams. For QPSK modulation, a numerical approach shows that the precoder design depends on the channel characteristics. Comparisons with maximum SNR strategy and other precoders based on criteria such as water-filling (WF), minimum mean square error (MMSE) and maximization of the minimum singular value of the global channel matrix are performed to illustrate the significant bit-error-rate (BER) improvement of the proposed precoder.