  Topological electronic structures are key to the topological classification
of quantum materials and play an important role in their physical properties
and applications. Recently, SnSe has attracted great research interests due to
its superior thermoelectric performance. However, it's topological nature has
long been ignored. In this work, by combining synchrotron-based angle-resolved
photoemission spectroscopy and ab-initio calculations, we systematically
investigated the topological electronic structure of orthorhombic SnSe. By
identifying the continuous gap in the valence bands due to the band inversion
and the topological surface states on its (001) surface, we establish SnSe as a
strong topological insulator. Furthermore, we studied the evolution of the
topological electronic structure and propose the topological phase diagram in
SnSe1-xTex. Our work reveals the topological non-trivial nature of SnSe and
provides new understandings of its intriguing transport properties.
