Short-wave infrared (SWIR) light sources featuring large spectral bandwidth and high luminescence efficiency have found wide applications in the fields of night vision, non-destructive detection, covert information identification, and biomedical imaging. In particular, phosphor-converted SWIR emitters are attracting much attention owing to their simple processing conditions, low product price, compact design, and tunable luminescence feature. However, discovering phosphor materials that emit completely in the SWIR spectral region (>900 nm) when excited by blue light remains a great challenge. Herein, we report a novel Yb-based inorganic phosphor, KYbP₂O₇:Cr³⁺, exhibiting pure and efficient SWIR photoluminescence in the wavelength range of 900–1200 nm upon blue light excitation. The formation of Cr³⁺–Yb³⁺ redox ion pairs results in efficient energy transfer from Cr³⁺ to Yb³⁺ in the KYbP₂O₇:Cr³⁺ phosphor. This new phosphor has a relatively high photoluminescence quantum efficiency of 51.4% and good thermal stability, maintaining 55.6% of the room temperature luminescence intensity at 120 °C. The as-fabricated SWIR-LED prototype devices through a combination of the optimized phosphor with blue LED chips can generate a maximum SWIR output power of 8.0 mW at 150 mA drive current, demonstrating great potential for practical applications. This work will open a new research avenue for the development of efficient, stable, and cost-effective phosphor-converted SWIR emitters.