The first $\ensuremath{\gamma}$-ray spectroscopy of $^{52}\mathrm{Ar}$, with the neutron number $N=34$, was measured using the $^{53}\mathrm{K}(p,2p)$ one-proton removal reaction at $\ensuremath{\sim}210\text{ }\text{ }\mathrm{MeV}/\mathrm{u}$ at the RIBF facility. The ${2}_{1}^{+}$ excitation energy is found at 1656(18) keV, the highest among the Ar isotopes with $N>20$. This result is the first experimental signature of the persistence of the $N=34$ subshell closure beyond $^{54}\mathrm{Ca}$, i.e., below the magic proton number $Z=20$. Shell-model calculations with phenomenological and chiral-effective-field-theory interactions both reproduce the measured ${2}_{1}^{+}$ systematics of neutron-rich Ar isotopes, and support a $N=34$ subshell closure in $^{52}\mathrm{Ar}$.