Altermagnetism provides a promising platform for valleytronics applications due to its symmetry-protected spin-valley locking. While uniaxial strain and in-plane magnetization have recently emerged as novel routes to induce valley polarization in altermagnets, a unified theoretical framework elucidating their microscopic mechanisms remains absent. Here, we use a tight-binding (TB) model to show that both uniaxial strain and in-plane magnetization can induce valley polarization in two-dimensional (2D) tetragonal altermagnets by breaking the $C_{4z}\mathcal{T}$ symmetry, although driven by distinct mechanisms. Specifically, uniaxial strain tunes valley polarization by modulating anisotropic hopping, yielding a monotonic dependence whose sign can be reversed by switching the strain axis ($x/y$) or type (compressive/tensile). In contrast, in-plane magnetization generates valley polarization via weak ferromagnetism, exhibiting a characteristic $cos2\varphi$ dependence determined by the spin direction. Crucially, these model predictions are confirmed by first-principles calculations on a $d$-wave altermagnet monolayer ${\mathrm{Mo}}_2{\mathrm{Te}}_2\mathrm{O}$, demonstrating tunable valley polarization. Our findings establish a unified theoretical framework for the control of valley polarization in 2D tetragonal altermagnets, providing a foundation for the development of valleytronics.

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• Magnetism
• Valley degrees of freedom
• Valleytronics
• 2-dimensional systems
• Symmetries
• First-principles calculations
• Tight-binding model