The Diels‐Alder reaction of 1‐aminovinylphosphonate esters has been studied for the first time as a suitable procedure leading to quaternary carbocyclic α‐aminophosphonates. The reaction is influenced by steric effects at the phosphonate functionality (bulky groups hinder the reaction) and electronic effects at the amino group (electron‐withdrawing substituents increase the reaction rate). The exo/endo ratio is constant, and no influence of the solvent is observed. The experimental results have been rationalized by DFT methods.