$\alpha \text{−}{\mathrm{UTe}}_3$, a van der Waals (vdW) actinide compound with a monoclinic ${\mathrm{ZrSe}}_3$-type structure, is a narrow-gap semiconductor with $5f$ moments. ${}^{125}\mathrm{Te}$ NMR reveals strongly anisotropic, layer-confined spin fluctuations below $\sim 20\mathrm{K}$, with the $a$-axis component enhanced, and a signal wipeout at the antiferromagnetic (AFM) transition $T_{\mathrm{N}}=5\mathrm{K}$. Single-crystal neutron diffraction finds $\mathit{q}\approx (0.17,0.5,0)$ and a longitudinal sinusoidal modulation of $a$-axis moments (amplitude $\approx 0.8{\mu }_{\mathrm{B}}$) with AFM stacking along $b$. A crystal electric field singlet-singlet induced-moment framework accounts for the easy-axis anisotropy, the small heat-capacity anomaly at $T_{\mathrm{N}}$, the reduced ordered moment, and the exchange-driven selection of $\mathit{q}$ in this localized $5f$ vdW magnet, establishing a constrained exchange geometry stabilizing this in-plane incommensurate state.

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• Antiferromagnetism
• Magnetism
• Spin dynamics
• van der Waals systems
• Neutron diffraction
• Nuclear magnetic resonance
• Specific heat measurements