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+.