This paper reports on the diffuse reflection spectra, photoluminescence spectra and chromaticity of ytterbium in CaAlSiN₃ at room temperature. It can be excited efficiently over a broad spectral range between 280 and 580 nm and exhibits a single intense red emission at 629 nm with a full width at half maximum of 75 nm due to the electronic transitions from the excited state of 4f¹³5d to the ground state 4f¹⁴ of Yb²⁺. The low energy of Yb²⁺ emission in CaAlSiN₃ can be attributed to the large nephelauxetic effect and crystal field splitting due to the coordination of Yb²⁺ by nitrogen. This novel developed CaAlSiN₃:Yb²⁺, which is the first Yb²⁺-activated nitride red-emitting phosphor, has potential applications in spectral conversion materials for warm-white LEDs. The width of the emission band, Stokes shift and thermal quenching mechanism of Yb²⁺ in CaAlSiN₃ and (oxy)nitride-based phosphors are discussed and compared with those of Eu²⁺.