Monolayer magnet of $\mathit{XY}$ type composes a pivotal part of the two-dimensional magnetism. As an important suspected $\mathit{XY}$-type antiferromagnet, whether $1L \mathrm{NiP}{\mathrm{S}}_3$ exhibits magnetic order remains elusive. Herein by helicity-resolved Raman and ultrafast spectroscopy of $\mathrm{NiP}{\mathrm{S}}_3$ from bulk to monolayer, we find that $1L \mathrm{NiP}{\mathrm{S}}_3$ is magnetically ordered with a Berezinskii-Kosterlitz-Thouless (BKT) transition at $T_{\mathrm{BKT}}\approx 140$ K. We have also performed large-scale density-matrix renormalization-group calculations to verify the ground state zigzag antiferromagnetic order and Monte Carlo simulation to confirm the BKT-transition temperature in $1L \mathrm{NiP}{\mathrm{S}}_3$. Our investigations establish $1L \mathrm{NiP}{\mathrm{S}}_3$ to be an $\mathit{XY}$ antiferromagnet with a relatively high-temperature BKT transition, providing an important platform for investigating complex couplings and topological excitations.

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• BKT transition
• Magnetism
• Layered crystals
• Raman spectroscopy
• Ultrafast pump-probe spectroscopy