Herein we computationally studied the excited-state properties and decay dynamics of methyl-4-hydroxycinnamate (OMpCA) in the lowest three electronic states, that is, (1)pi pi*, (1)n pi*, and S-0 using combined MS-CASPT2 and CASSCF electronic structure methods. We found that one-water hydration can significantly stabilize and destabilize the vertical excitation energies of the spectroscopically bright (1)pi pi* and dark (1)n pi* excited singlet states, respectively; in contrast, it has a much smaller effect on the (1)pi pi* and (1)n pi* adiabatic excitation energies. Mechanistically, we located two (1)pi pi* excited-state relaxation channels. One is the internal conversion to the dark (1)n pi* state, and the other is the (1)pi pi* photoisomerization that eventually leads the system to a (1)pi pi*/S-0 conical intersection region, near which the radiationless internal conversion to the S0 state occurs. These two (1)pi pi* relaxation pathways play distinct roles in OMpCA and its two one-water complexes (OMpCA-W1 and OMpCA-W2). In OMpCA, the predominant (1)pi pi* decay route is the state-switching to the dark (1)n pi* state, while in one-water complexes, the importance of the (1)pi pi* photoisomerization is significantly enhanced because the internal conversion to the (1)n pi* state is heavily suppressed due to the one-water hydration.