People know little about the characteristics of the photonic crystals (PhCs) made of nanoscale building blocks, which can be quite distinct from the usual PhCs made of microscale or even larger units, because the unit size strongly affects the photonic structure. The PhCs made of fluorescent CuInSe₂ quantum dots (QDs) having an ultrasmall average size of 2.2–5.3 nm are studied. The experiments and hybrid density-functional theory calculation reveal that these QDs have defect-related fluorescence, and the light absorption exhibits an unexpected indirect-gap feature because of the discrete and small density of states at the direct-gap maximum. The frequency-domain electromagnetic calculations reveal that the PhCs made of the CuInSe₂ QDs have a remarkable out-of-plane bandgap in the extreme-UV (EUV) region. The frequency modes below and above the forbidden gap are separately air band and dielectric band modes. These multifunctional fluorescent semiconductor QD PhCs have application potential in nanoscale photonic devices and EUV photolithography.