  Monolayer graphene exhibits many spectacular electronic properties, with
superconductivity being arguably the most notable exception. It was
theoretically proposed that superconductivity might be induced by enhancing the
electron-phonon coupling through the decoration of graphene with an alkali
adatom superlattice [Profeta et al. Nat. Phys. 8, 131-134 (2012)]. While
experiments have indeed demonstrated an adatom-induced enhancement of the
electron-phonon coupling, superconductivity has never been observed. Using
angle-resolved photoemission spectroscopy (ARPES) we show that lithium
deposited on graphene at low temperature strongly modifies the phonon density
of states, leading to an enhancement of the electron-phonon coupling of up to
$\lambda\!\simeq\!0.58$. On part of the graphene-derived $\pi^*$-band Fermi
surface, we then observe the opening of a $\Delta\!\simeq\!0.9$ meV
temperature-dependent pairing gap. This result suggests for the first time, to
our knowledge, that Li-decorated monolayer graphene is indeed superconducting
with $T_c\!\simeq\!5.9 K$.
