The Advanced Television Systems Committee (ATSC) finalized its next-generation digital terrestrial broadcast standard in 2016, known as ATSC 3.0. In order to enhance spectral efficiency, the standard employs two-dimensional non-uniform constellations (2D-NUCs) for signal mapping. As a result, a 2D demapper, with computational complexity much higher than one-dimensional (1D) demappers, has to be designed for the receiver. In this paper, a suboptimal low-complexity 2D-demapping method is proposed for ATSC 3.0. The proposed method takes advantage of the 2D-NUC's feature that constellation points with same most significant bits tend to locate in the same subregion. Simulation results show that compared with the conventional Max-Log-MAP 2D demapping, the proposed method achieves at least 74% complexity reduction with negligible performance loss. Moreover, compared with another 2D demapper specially designed for ATSC 3.0, the proposed method dispenses with high-complexity exponential and logarithmic operations at a small performance cost of at most 0.14 dB. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.