  The rare-earth monopnictide LaBi exhibits exotic magneto-transport properties
including an extremely large and anisotropic magnetoresistance. Experimental
evidence for topological surface states is still missing although band
inversions have been postulated to induce a topological phase in LaBi. By
employing angle-resolved photoemission spectroscopy (ARPES) in conjunction with
$ab~initio$ calculations, we have revealed the existence of surface states of
LaBi through the observation of three Dirac cones: two coexist at the corners
and one appears at the center of the Brillouin zone. The odd number of surface
Dirac cones is a direct consequence of the odd number of band inversions in the
bulk band structure, thereby proving that LaBi is a topological, compensated
semi-metal, which is equivalent to a time-reversal invariant topological
insulator. Our findings provide insight into the topological surface states of
LaBi's semi-metallicity and related magneto-transport properties.
