  Although the concept of Luttinger liquid (LL) that describes a
one-dimensional (1D) interacting fermion system collapses in higher dimensions,
it has been proposed to be closely related to many mysteries including the
normal state of cuprate superconductor, unconventional metal, and quantum
criticality. Therefore, the generalization of LL model to higher dimensions has
attracted substantial research attention. Here we systematically investigate
the electronic structure of a quasi-2D compound {\eta}-Mo4O11 using
high-resolution angle-resolved photoemission spectroscopy and ab-initio
calculation. Remarkably, we reveal a prototypical LL behavior originating from
the crossing quasi-1D chain arrays hidden in the quasi-2D crystal structure.
Our results suggest that {\eta}-Mo4O11 materializes the long sought-after
crossed LL phase, where the orthogonal orbital components significantly reduce
the coupling between intersecting quasi-1D chains and therefore maintain the
essential properties of LL. Our finding not only presents a realization of 2D
LL, but also provides a new angle to understand non-Fermi liquid behaviors in
other 2D and 3D quantum materials.
