A sulfahydantoin (3‐oxo‐1,2,5‐thiadiazolidine 1,1‐dioxides) motif is used as a new type of peptidic constraint to lock two consecutive amide nitrogens by a sulfonyl bridge. The 5‐membered heterocyclic motif was prepared starting from proteogenic and synthetic amino acids and chlorosulfonyl isocyanate. Constrained dipeptides were obtained under alkaline conditions (methoxide or tert‐butoxide) by cyclization of symmetric and dissymmetric sulfamides. The absolute configuration of the chiral centers for the derivative L‐Phe‐D‐Ala, a congener of the series, was established by X‐ray diffraction crystallographic analysis. In addition, the chemo‐, regio‐, and stereoselectivities of the reactions were studied. In the acylated derivatives, the sulfahydantoin constraint induces a unique backbone conformation with coplanarity of two consecutive peptide bonds.