  Topological insulators (TIs) host novel states of quantum matter,
distinguished from trivial insulators by the presence of nontrivial conducting
boundary states connecting the valence and conduction bulk bands. Up to date,
all the TIs discovered experimentally rely on the presence of either time
reversal or symmorphic mirror symmetry to protect massless Dirac-like boundary
states. Very recently, it has been theoretically proposed that several
materials are a new type of TIs protected by nonsymmorphic symmetry, where
glide-mirror can protect novel exotic surface fermions with hourglass-shaped
dispersion. However, an experimental confirmation of such new nonsymmorphic TI
(NSTI) is still missing. Using angle-resolved photoemission spectroscopy, we
reveal that such hourglass topology exists on the (010) surface of crystalline
KHgSb while the (001) surface has no boundary state, which is fully consistent
with first-principles calculations. We thus experimentally demonstrate that
KHgSb is a NSTI hosting hourglass fermions. By expanding the classification of
topological insulators, this discovery opens a new direction in the research of
nonsymmorphic topological properties of materials.
