Red-emitting (La,Eu)2O2SO4 phosphors have been successfully prepared using the layered hydroxide of (La,Eu)2(OH)4SO4·2H2O as the precursor. The precursor compound was firstly crystallized via hydrothermal reaction (100 °C and pH = 9.0) as well-dispersed nanoplates, followed by dehydration and dehydroxylation in the 400–1200 °C temperature range in ambient air to yield (La,Eu)2O2SO4. The phosphors show intense red emissions originated from the f–f transitions of Eu3+, dominantly peaking at 617 nm, under O–Eu charge transfer excitation at 284 nm. The optimal Eu3+ content was experimentally determined to be 5 at.%, agreeing well with theoretical analysis, and the concentration quenching of luminescence was suggested to be due to exchange interactions. Fluorescence decay analysis indicates that a higher calcination temperature or Eu3+ content would decrease the lifetime of the 617 nm emission.