Chlorophyll a (Chl a) is the most abundant pigment on earth. In all plants, algae, and cyanobacteria, it plays a pivotal role as an antenna and reaction center pigment in the primary steps of photosynthesis. In the past, a true three-dimensional (3D) experimental determination of the Q(y) electronic transition dipole moment orientation could not be obtained. With combined femtosecond polarization resolved VIS pump-IR probe experiments and theoretical calculations of the infrared transition dipole moments (tdm's) in the electronic ground state, we determined the 3D orientation of the Q(y) electronic tdm of Chl a within the molecular structure. Polarization resolved experiments provided angles of the Q(y) electronic tdm with three different infrared tdm's, whose orientations within the molecular structure were taken from our theoretical calculations. The orientation of the Q(y) tdm results from the intersection of all three angles and was found to have an angle of (78 +/- 3)degrees with the x-axis, (12 +/- 3)degrees with the y-axis, and (86 +/- 2)degrees with the z-axis.