A novel III-Nitride digital alloy (DA) with ultra-broadband optical gain is proposed. Numerical analysis shows a 50-period InN/GaN DA yields minibands that are densely quantized by numerous confined states. Interband transitions between the conduction and valence minibands create ultra-broadband optical gain spectra with bandwidths up to ~1 μm that can be tuned from the red to infrared. In addition, the ultra-broadband optical gain, bandwidth, and spectral coverage of the III-Nitride DA is very sensitive to layer thickness and other structural design parameters. This study shows the promising potential of the III-Nitride DAs with tunable ultra-broadband interband optical gain for use in semiconductor optical amplifiers and future III-Nitride photonic integration applications.