Femtosecond UV-vis pump-probe spectroscopy was employed to study the acid effect on curcumin in the excited state. Curcumin in solutions of weak acids was found to be a photobase forming a protonated curcumin within a few tens of picoseconds from the time of excitation. The excited-state protonation reaction is also observed in the steady-state emission spectrum as a new red emission band with a maximum at 620 nm in the presence of weak acids. The transient pump-probe spectrum consists of four spectral bands, two emission bands, and two absorption bands. We assign a transient absorption band at similar to 600 nm and an emission band at similar to 540 nm to the neutral ROH form of curcumin. An absorption band at similar to 500 nm and an emission band at 620 nm are assigned to the protonated ROH2+ form of curcumin.