The external electric field effects on the steady-state electronic spectra and excited-state dynamics were investigated for 4′-N,N-(dimethylamino)-3-hydroxyflavone (DMHF) in a poly(methyl methacrylate) (PMMA) film. In the steady-state spectrum, dual emission was observed from the excited states of the normal (N∗) and tautomer (T∗) forms. Application of an external electric field of 1.0 MV·cm-1 enhanced the N∗ emission and reduced the T∗ emission, indicating that the external electric field suppressed the excited-state intramolecular proton transfer (ESIPT). The fluorescence decay profiles were measured for the N∗ and T∗ forms. The change in the emission intensity ratio N∗/T∗ induced by the external electric field is dominated by ESIPT from the Franck-Condon excited state of the N∗ form and vibrational cooling in potential wells of the N∗ and T∗ forms occurring within tens of picoseconds. Three manifolds of fluorescent states were identified for both the N∗ and T∗ forms. The excited-state dynamics of DMHF in PMMA films has been found to be very different from that in solution due to intermolecular interactions in a rigid environment.