Diamines, such as 1,3-diaminopropane (Dap), 1,4-diaminobutane (putrescine, or Put), and 1,5-diaminopentane (cadaverine), are essential organic bases for plants. Such “biogenic diamines” are abundantly present in oak tree flowers and are emitted into the atmosphere. We sampled atmospheric diamines in a typical broadleaf tree forest and chestnut grove during blossom season and carried out a chromatography-mass spectrometry analysis. Amide derivatives of Dap and Put, 3-aminopropanamide and 4-aminobutanamide, respectively, were found in higher atmospheric concentrations than their source diamine. Because their concentrations in oak tree flowers were very low, we deduced that these aminoamides were produced after Dap and Put, respectively, were released into the atmosphere. Time series of biogenic diamine and aminoamide concentrations showed evidence of diurnal variations. Concentrations were lower during daytime in sunny conditions, suggesting relatively fast photochemical decomposition. Biogenic diamines and aminoamides bond more easily than monoamines with water vapor and sulfuric acid in the atmosphere. Biogenic diamines and aminoamides may play a role in the formation of new particles and biogenic secondary organic aerosols in the forest atmosphere during spring and early summer.