Large magnetic anisotropy and high spin state in two-dimensional <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>MnIn</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi>Se</mml:mi> <mml:mn>3</mml:mn> </

Two-dimensional (2D) van der Waals magnetic materials hold great potential for advancing ultraminiature spintronic and information storage devices. Nevertheless, existing candidates are often constrained by their low magnetic anisotropy energy (MAE) or relatively small magnetic moments. In this work, we predict monolayer ${\mathrm{MnIn}}_{2}{\mathrm{Se}}_{3}{\mathrm{I}}_{2}$ as a 2D magnetic material, exhibiting a substantial local moment of 4.41 \ensuremath{\mu}${}_{\mathrm{B}}/\mathrm{Mn}$ together with a remarkably large MAE of up to $931\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{eV}/\mathrm{Mn}$. Phonon spectra and molecular dynamics simulations confirm that the crystal structure of ${\mathrm{MnIn}}_{2}{\mathrm{Se}}_{3}{\mathrm{I}}_{2}$ is both dynamically and thermodynamically stable. Owing to the pronounced anisotropy in magnetic exchange interactions along the two orthogonal in-plane directions, the system undergoes two distinct magnetic phase transitions at different temperatures. Overall, this study not only identifies ${\mathrm{MnIn}}_{2}{\mathrm{Se}}_{3}{\mathrm{I}}_{2}$ as a promising 2D magnetic candidate for future applications, but also elucidates the intrinsic correlation between its structural stability and magnetic tuning mechanisms.