Generating and controlling spin current in miniaturized magnetic quantum devices remains a central objective of spintronics, due to its potential to enable future energy-efficient information technologies. Among the existing magnetic phases, altermagnetism has recently emerged as a highly promising platform for spin current generation and control, going beyond ferromagnetism and antiferromagnetism. Here, we propose a symmetry-allowed spin photovoltaic effect in two-dimensional (2D) altermagnetic semiconductors that enables predictable control of giant spin injection currents. Distinct from parity-time $\left(\mathcal{PT}\right)$-antiferromagnets, Janus altermagnetic semiconductors generate not only shift current but also a unique injection current with spin momentum locked in a specific direction under linearly polarized light—a mechanism absent in $\mathcal{PT}$-antiferromagnets. Through symmetry analysis and first-principles calculations, we identify Janus ${\mathrm{Fe}}_2\mathrm{SSeO}$ as a promising candidate. Specifically, the monolayer ${\mathrm{Fe}}_2\mathrm{SSeO}$ exhibits a polarization-dependent injection conductivity reaching $\sim 1200\mu \mathrm{A}/{\mathrm{V}}^2\hslash /2e$, and the giant spin injection current can be effectively switched by rotating the magnetization direction and engineering strains. These findings underscore the potential of 2D altermagnets in spin photovoltaics and open avenues for innovative quantum devices.

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• Electrical properties
• Electronic structure
• Magnetism
• Photoconductivity
• First-principles calculations