  Topological nodal line semimetals, a novel quantum state of materials,
possess topologically nontrivial valence and conduction bands that touch at a
line near the Fermi level. The exotic band structure can lead to various novel
properties, such as long-range Coulomb interaction and flat Landau levels.
Recently, topological nodal lines have been observed in several bulk materials,
such as PtSn4, ZrSiS, TlTaSe2 and PbTaSe2. However, in two-dimensional
materials, experimental research on nodal line fermions is still lacking. Here,
we report the discovery of two-dimensional Dirac nodal line fermions in
monolayer Cu2Si based on combined theoretical calculations and angle-resolved
photoemission spectroscopy measurements. The Dirac nodal lines in Cu2Si form
two concentric loops centred around the {\Gamma} point and are protected by
mirror reflection symmetry. Our results establish Cu2Si as a new platform to
study the novel physical properties in two-dimensional Dirac materials and
provide new opportunities to realize high-speed low-dissipation devices.
