Erbium ions (${\mathrm{Er}}^{3+}$) provide a telecom-band optical transition with strong magnetic-dipole character, making them attractive for quantum communication and spin-photon interfaces. Identifying host environments that combine low decoherence with photonic compatibility, however, remains a central challenge. Here we investigate ${\mathrm{Er}}^{3+}$ emission in tungsten disulfide (${\mathrm{WS}}_2$) flakes, a layered host offering low nuclear-spin density and narrow telecom emission. Using time- and polarization-resolved photoluminescence under modest magnetic fields ($<0.2\mathrm{T}$), we observe pronounced dimming, lifetime extension, and rotation of the emission dipole when the field has an out-of-plane component, whereas in-plane fields produce little change. Effective model calculations of ${\mathrm{Er}}^{3+}$ in monolayer ${\mathrm{WS}}_2$ parametrized from density functional theory suggest that these effects arise primarily from Zeeman-induced mixing of near-degenerate crystal-field (CF) sublevels, which modifies the magnitude and orientation of the optical transition dipole moments. Comparative measurements in flakes of different thickness and numerical estimates of the local density of optical states further point to a secondary contribution from dipole coupling to the anisotropic photonic environment of thin ${\mathrm{WS}}_2$ layers. These findings identify layered ${\mathrm{WS}}_2$ as a platform where magnetic fields can tune telecom emission through an interplay of crystal-field physics and anisotropic photonic coupling.

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• Magneto-optics
• Optical quantum information processing
• Optoelectronics
• Photonics
• Quantum communication
• Rare-earth doped crystals
• Transition metal dichalcogenides
• Infrared techniques