Europium doped strontium iodide (SrI2:Eu2+) is one of the cutting-edge halide scintillators for gamma-ray spectroscopy applications due to its low radioactive background, ultrahigh light yield (70,000–100,000 photons/MeV), and excellent gamma-ray energy resolution (2.6%–3% at 662 keV). In this work, we report an improvement in the scintillation characteristics of SrI2:Eu2+ single crystals via tetravalent ion codoping, namely Zr4+. High quality SrI2:Eu2+ single crystals codoped with 0, 0.05, 0.1, and 0.2 at.% Zr4+ were grown by the Multi-ampoule Bridgman method. All of the as-grown crystals are transparent, crack-and inclusion-free. They have high optical quality with a transmittance of over 70%. The effects of Zr4+ codoping on spectral properties and decay kinetics of SrI2:Eu2+ under optical, and x- and gamma-ray excitation are studied. An enhancement of gamma-ray spectroscopic resolution from 2.8 to 2.5% at 662 keV and 5.4 to 5% at 122 keV is achieved upon Zr4+ codoping with a slightly enhanced light yield of 95,000 photons/MeV.