In the field of solid-state luminescence, Cu²⁺ has long been widely acknowledged for its capacity to emit infrared light. However, the occurrence of visible emission from Cu²⁺ ions had been infrequently observed and reported. In this study, we made an intriguing discovery by examining the behavior of Cu²⁺ within an irregular coordination environment of Ba in BaGa₂O₄. When excited by UV light, Cu²⁺ unexpectedly gave a vibrant yellow–red emission, covering a wavelength range spanning from 500 to 750 nm. More noteworthy, by simply manipulating the excitation wavelength or adjusting the temperature, the peak wavelength of the emission could be effectively tuned from approximately 600 to 660 nm, which could be attributed to the luminescence nature of the charge transfer (CT) between O²⁻ and Cu²⁺. Moreover, the phosphor material displayed a remarkable persistent luminescence (PerL) lasting up to 12 h after UV light excitation. Through thermoluminescence (TL) measurements and first-principle calculations, we found that the intrinsic defects, such as vacancies of oxygen and gallium (V_O and V_Ga^″), played important roles for the PerL phenomena. These findings highlighted the exceptional tunability and PerL properties of BaGa₂O₄:Cu²⁺. Our study provided a new potential guideline for the design of Cu²⁺-activated phosphors in visible region, and opened up new avenues for the research in related functional luminescence materials.