In this work, we present a comprehensive investigation of the decomposition of tamsulosin. Experiments were performed in an aqueous solution through photolysis with a 254 nm light source. In general, tamsulosin degradation was observed (presenting a rate of 35.4%, in 180 min), demonstrating that direct photoinduced decomposition is efficient. The simpler uncatalyzed process worked while previous experiments using solid catalysts showed negligible degradation. The kinetic constant of the photolysis process (17.6 ×10⁻³ min⁻¹) was determined, considering first order kinetics, by fitting plots of ln([Tam]_t −[Tam]_∞)/ln([Tam]₀ −[Tam]_∞) versus irradiation time, where [Tam]_t is the concentration of tamsulosin at time t, and [Tam]_∞ is the limiting concentration of the reacting system. In addition, physical insights regarding tamsulosin photoexcitation and photolysis were obtained through quantum chemistry computations. Density functional theory (DFT) was used to investigate the ground state while its time-dependent formalism (TD-DFT) was used for determining vertical excitation energies and generalized oscillator strengths(GOSs). All the computations were undertaken at the CAM-B3LYP/6-311++G(d,p) level of theory, in water. Regarding the excited states, all five lowest-lying states were determined to have non-zero GOSs, with S₃ (found at 5.73 eV (∼216 nm) at the TD-DFT/CAM-B3LYP/6–311++G(d,p) level of theory) presenting the highest GOS (0.1157) among all. Hence, these accessible states may be assigned as those responsible for the photoinduced degradation observed experimentally. Moreover, the computational results suggest tamsulosin undergoes photodecomposition through excited state chemical reaction rather than via a direct photolysis path.