LAUSR

Topological crystalline insulators (TCIs) with hourglass fermion surface state have attracted a lot of attention and are further enriched by crystalline symmetries and magnetic order. Here, the emergence of hourglass fermion surface state and exotic phases in the newly discovered, air‐stable ErAsS single crystals are shown. In the paramagnetic phase, ErAsS is expected to be a TCI with hourglass fermion surface state protected by the nonsymmorphic symmetry. Dirac‐cone‐like bands and nearly linear dispersions in large energy range are experimentally observed, consistent well with theoretical calculations. Below TN ≈ 3.27 K, ErAsS enters a collinear antiferromagnetic state, which is a trivial insulator breaking the time‐reversal symmetry. An intermediate incommensurate magnetic state appears in a narrow temperature range (3.27–3.65 K), exhibiting an abrupt change in magnetic coupling. The results reveal that ErAsS is an experimentally available TCI candidate and provide a unique platform to understand the formation of hourglass fermion surface state and explore magnetic‐tuned topological phase transitions.