Altermagnets represent a newly identified third class of collinear magnets and have recently emerged as a focal point in condensed matter physics. In this work, through first-principles calculations and theoretical analysis, we identify monolayer ${\mathrm{Fe}}_2\mathrm{Mo}{X}_4$ ($X$ = S, Se, Te) and ${\mathrm{Fe}}_2{\mathrm{WTe}}_4$, a class of iron-based transition metal chalcogenides, as promising altermagnetic materials. These systems are found to be semiconductors exhibiting spin splitting in their nonrelativistic band structures, indicative of intrinsic altermagnetic ordering. Remarkably, their valence bands feature a pair of valleys at the time-reversal-invariant momenta $X$ and $Y$ points. Unlike conventional valley systems, these valleys are related by crystal symmetries rather than time-reversal symmetry. We investigate valley-dependent physical phenomena in these materials, including Berry curvature and optical circular dichroism, revealing strong valley-contrasting behavior. Furthermore, we investigate the effect of uniaxial strain and show that it effectively lifts the valley degeneracy, resulting in pronounced valley polarization. Under hole doping, this strain-induced asymmetry gives rise to a piezomagnetic response. We also explore the generation of anisotropic noncollinear spin currents in these systems, expanding the scope of their spin-related functionalities. Our findings unveil rich valley physics in monolayer ${\mathrm{Fe}}_2\mathrm{Mo}{X}_4$ ($X$ = S, Se, Te) and ${\mathrm{Fe}}_2{\mathrm{WTe}}_4$, highlighting their significant potential for applications in valleytronics, spintronics, and multifunctional nanoelectronic devices.

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• Electronic structure
• First-principles calculations
• Magnetism
• Valleytronics
• Crystal structures
• Magnetic systems
• Density functional theory