Altermagnets (AMs) combine fully compensated magnetization with momentum-dependent spin splitting, yet intrinsic altermagnetic materials exhibiting exceptional valley characteristics remain scarce. Here, we identify monolayer titanium-based chalcogenide halides, ${\mathrm{Ti}}_2{X}_{2}Y$ ($X=\mathrm{F}$, Cl, Br, I; $Y$ = O, S, Se, Te), as a new family of two-dimensional (2D) altermagnetic valley materials. These monolayers exhibit robust $d$-wave altermagnetic order, semiconducting band gaps, and pronounced spin-polarized valley characteristics. We show that uniaxial strain breaks the valley degeneracy, inducing giant valley splitting together with a tunable piezomagnetic response. An in-plane electric field generates noncollinear spin currents, while spin-orbit coupling gives rise to the anomalous Hall effect, valley-selective linear dichroism, and the magneto-optical Kerr effect. These findings establish ${\mathrm{Ti}}_2{X}_{2}Y$ monolayers as a versatile platform for exploring spin- and valley-dependent electronic, optical, and transport phenomena in 2D altermagnets.

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• Electrical conductivity
• Electrical properties
• Electronic structure
• Magnetism
• Valley degrees of freedom
• Valleytronics
• First-principles calculations