We present a novel approach to scintillating crystal engineering through the incorporation of fluorescein molecules as a dopant into a cesium iodide (CsI) matrix. This work provides a comprehensive preparation method and characterization of the resulting hybrid material for applications in ionizing radiation detectors. Comprehensive characterization included powder X-ray diffraction (PXRD) to assess the dopant’s influence on the CsI lattice, followed by spectroscopic measurements (absorption, fluorescence, and fluorescence lifetime) to determine the optical properties and fast response time. Crucially, the quantum yield was also measured as a metric of energy conversion efficiency. The results demonstrate that the inclusion of fluorescein in the CsI matrix leads to a new scintillator, exhibiting superior properties compared to conventional materials such as CsI(Tl) or NaI(Tl). The presented research is the first step toward developing a new generation of scintillating detectors characterized by an optimal emission wavelength, high quantum yield, and fast response time, which are critical for applications in nuclear medicine, particle physics, and security systems.