We investigated the fundamental photophysics and photochemistry of a cationic dye rhodamine 800 (R800) in water using steady-state and ultrafast time-resolved spectroscopies. In the ground state, the monomer and dimer coexist in equilibrium, which causes significant concentration dependence of UV-visible (vis) absorption spectra. We determined the equilibrium constant as well as the molar absorption spectra of the monomer and dimer from a global fitting analysis of the UV-vis spectra. The obtained pure dimer spectrum indicates that it is a nonparallel H-dimer. In contrast to the absorption spectra, the steady-state fluorescence spectra do not show any noticeable concentration dependence. The fluorescence lifetime was determined as 0.73 ns regardless of the concentration, and the fluorescence of R800 in water was solely attributed to the monomer. In femtosecond time-resolved absorption measurements, we observed the S-n <- S-1 absorption bands of the monomer and the dimer, as well as the ground-state bleaching signals. It was found that the S-1 dimer dissociates to produce the S-1 monomer (and the S-0 monomer) or relaxes to the So dimer with a time constant of as short as 3.0 ps, which brings about the absence of dimer fluorescence.