Spatial diversity cross-polar approach to fast electromagnetic imaging of millimeter-wave chipless radio-frequency identification (RFID) tags is proposed. Slow imaging in the synthetic aperture radar (SAR) technique due to the moving reader antenna is replaced by a stationary multiple-input multiple-output (MIMO) array antenna. The proposed MIMO technique requires a large number of antenna elements, which impedes practical implementation of the reader. An optimized MIMO system through a genetic algorithm halves the number of antenna elements and successfully replaces the conventional SAR. The optimized antenna system combines with the original spatial diversity approach to provide a low-cost solution for chipless RFID tagging with enhanced content capacity. The system successfully decodes a 60-GHz 17-b chipless tag in an area that is one fifth of a credit card. The tag can be printed with a low-resolution printer and read on metal and liquid containers.