Sub-5-fs spectroscopy of porphyrin J-aggregates reveals a coherent molecular vibration coupled to the Frenkel exciton. The bleaching and induced absorption signals show synchronous oscillations with the frequency of 244 cm(-1). The coherent oscillation is explained by a modulated transition dipole moment, which is due to a dynamic intensity borrowing from the intense B-transition to the weak Q-transition through the ruffling mode with the 244 cm(-1)-frequency.