Nonlinear optical characterization of nanostructured layered transition metal dichalcogenides (LTMDs) is of fundamental interest for basic knowledge and applied purposes. In particular, second-order optical nonlinearities are the basis for second harmonic generation as well as sum or difference frequency generation and have been studied in some 2D TMDs, especially in those with a semiconducting character. Here we report, for the first time, on the second-order nonlinearity of the semi-metallic ZrTe₂ monolayer in acetonitrile suspension (concentration of 4.9 × 10¹⁰ particles per cm³), synthesized via a modified redox exfoliation method and characterized using the Hyper–Rayleigh scattering technique in the nanosecond regime. The orientation-averaged first-hyperpolarizability was found to be β(2ω) = (7.0 ± 0.3) × 10⁻²⁴ esu per ZrTe₂ monolayer flake, the largest reported so far. Polarization-resolved measurements were performed in the monolayer suspension and indicate the dipolar origin of the generated incoherent second harmonic wave.