Synthesized CuIr2(Te1-xSex)4 samples were comprehensively characterized using various techniques. At ambient pressure, CuIr2Te4 undergoes an anomalous phase transition at Ts∼250K with a large thermal hysteresis of ΔTs ∼40K and a superconducting transition at Tc∼3K during magnetization and electrical-resistivity measurements. We determined this transition to be a first-order structural phase transition from high-temperature hexagonal (P3¯m1) to low-temperature triclinic (P1¯) symmetry. Both external physical pressure and chemical doping largely enhance Ts and ΔTs and suppress Tc, thereby indicating a strong correlation between Ts and Tc. Critically, no superlattice is observed below Ts as per the electron-diffraction examinations. Thus, the anomalous phase transition at Ts, which exhibits a large thermal hysteresis in resistivity and magnetization measurements, is structural rather than a charge-density-wave formation.