Complexation of Am³⁺ and Ln³⁺ (La³⁺, Eu³⁺, and Er³⁺) with two bis(1,2,4‐triazinyl)bipyridine (C₂BTBP, C₅BTBP) derivatives has been studied in acetonitrile medium with use of various experimental techniques such as electrospray ionization mass spectrometry (ESI‐MS), time‐resolved fluorescence spectroscopy (TRFS), UV/Vis spectrophotometry, and solution calorimetry. Metal–ligand stoichiometries and conditional stability constants of these complexes were determined. To the best of our knowledge, this is the first report on the complexation of Am³⁺ with any of the BTBP derivatives with use of UV/Vis spectrophotometric titration to determine the conditional stability constants. Density functional theory (DFT) calculations are carried out on the An³⁺ (U³⁺ and Am³⁺) and Ln³⁺ (La³⁺, Nd³⁺, Eu³⁺, Er³⁺, and Lu³⁺) complexes of BTBP in order to understand the difference between the bonding in actinide and lanthanide complexes. The results indicate a stronger covalent interaction in the An–N bonds as compared to the Ln–N bonds, which leads to an actinide selectivity of this class of ligands.