There is provided a light source device, a headlight, a display apparatus, and an illumination apparatus having excellent heat dissipation. The light source device includes a substrate, a phosphor, a light emitting element, and a wavelength-selective reflecting member. The phosphor is disposed in contact with the substrate. The light emitting element emits excitation light for exciting the phosphor. The wavelength-selective reflecting member partially reflects the excitation light emitted from the light emitting element to be guided to the phosphor and transmits fluorescence emitted from the phosphor by excitation caused by incidence of the excitation light and the excitation light reflected by the phosphor.