Iterative detection and decoding (IDD), combined with spatial-multiplexing multiple-input multiple-output (MIMO) transmission, is a key technique to improve spectral efficiency in wireless communications. In this paper we present the—to the best of our knowledge—first complete silicon implementation of a MIMO IDD receiver. MIMO detection is performed by a multi-core sphere decoder supporting up to 4×4 as antenna configuration and 64-QAM modulation. A flexible low-density parity check decoder is used for forward error correction. The 65 nm CMOS ASIC has a core area of 2.78 mm2 . Its maximum throughput exceeds 1 Gbit/s, at less than 1 nJ/bit. The MIMO IDD ASIC enables more than 2 dB performance gains with respect to non-iterative receivers.