We report single-molecule conformational transitions with joint angstrom-femtosecond resolution by irradiating the junction of a scanning tunneling microscope with femtosecond laser pulses. An isolated pyrrolidine molecule adsorbed on a Cu(001) surface undergoes reversible transitions between two conformational states. The transition rate decays in time and exhibits sinusoidal oscillations with periods of specific molecular vibrations. The dynamics of this transition depends sensitively on the molecular environment, as exemplified by the effects of another molecule in proximity.