A low-complexity sparse absolute-term based nonlinear equalizer (AT-NLE) is proposed to eliminate the nonlinear signal distortions for intensity modulation and direct detection (IM/DD) systems. By performing the orthogonal matching pursuit (OMP) algorithm to adaptively obtain the significant kernels of both the linear and absolute terms, the computational complexity of the proposed sparse AT-NLE is dramatically reduced and independent of the memory length. The performance of the proposed sparse AT-NLE is experimentally evaluated in a C-band 56-Gbit/s four-level pulse-amplitude modulation (PAM-4) system over a 30-km standard single-mode fiber (SSMF). Experimental results show that compared with the conventional diagonally-pruned Volterra nonlinear equalizer (DP-VNLE) or DP-AT-NLE, the proposed sparse AT-NLE saves 77.7% or 76% real-valued multiplications when their achieved bit error ratios (BERs) are similar. Meanwhile, the proposed sparse AT-NLE reduces the computational complexity by > 28% compared to the sparse DP-VNLE at a BER of 5 × 10-4. The proposed low-complexity sparse AT-NLE shows great potential for high-performance and low-cost IM/DD optical transmission systems.