Electronic interaction between the total angular momentum (J) of the 4f electronic system and an angular momentum (L) of the photoexcited cyclic pi system in a terbium(III) monoporphyrinato complex with a capping ligand cyclen, [Tb(TPP)(cyclen)]Cl (TPP = 5,10,15,20-tetraphenylporphyrinato; cyclen = 1,4,7,10-tetraazacyclododecane), have been investigated by varied-temperature and varied-magnetic-field magnetic circular dichroism (MCD) spectroscopy. Three MCD A-term patterns which correspond to B(0,0), Q(0,0), and Q(1,0) absorption bands were observed for the complex which was incorporated into a thin film of poly(methyl methacrylate), PMMA, as well as that in solution phase. The A-term patterns in the Soret band region and Q-band region were found to show opposite behavior with changes in temperature and magnetic field. This finding clearly indicated not only the presence of the J-L interaction but also the different directions of the J-L interaction; i.e., J and L interact ferromagnetically in the Q(0,0) and Q(1,0) states, while antiferromagnetically in the B(0,0) state. The magnitude of the interaction was determined by simulation-based fitting to experimental H-1/D-0 ratios, where H-1 refers to the A-term intensity and D-0 to the oscillator strength of the band. The first ab initio study was also carried out for further insight into the interaction. From the computational study, the relationship between the orbital angular momentum of pi electronic system, L(pi), and the orbital angular momentum of the 4f system, L(f), was clearly revealed. A computational approach to determine the magnitude and the direction of J-L interaction was established.