  The search for metallic boron allotropes has attracted great attention in the
past decades and recent theoretical works predict the existence of metallicity
in monolayer boron. Here, we synthesize the \b{eta}12-sheet monolayer boron on
a Ag(111) surface and confirm the presence of metallic boron-derived bands
using angle-resolved photoemission spectroscopy. The Fermi surface is composed
of one electron pocket at the S point and a pair of hole pockets near the X
point, which is supported by the first-principles calculations. The metallic
boron allotrope in \b{eta}12 sheet opens the way to novel physics and chemistry
in material science.
