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Bogoliubov Fermi surface (BFS) is an unprecedented superconducting gap structure in which the nodal (zero-gap) region forms a two-dimensional surface rather than a line or point. Tetragonal Fe(Se,S) having a large residual density of states is a candidate for hosting BFSs. However, direct evidence has not yet been obtained. Here we report the unique momentum dependence of the superconducting gap in the hole band of FeSe0.78S0.22 by using high-energy-resolution laser-based angle-resolved photoemission spectroscopy. We find highly unusual gap anisotropy with a maximum gap of ~1 meV and widespread zero-gap regions with an accuracy of 200 μeV. Surprisingly, the gap structure breaks the four-fold rotational symmetry of the tetragonal crystal structure. These results reveal the formation of anomalous two-fold symmetric nodal planes, providing evidence for emergent nematic BFSs.

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