A new class of peptidomimetic is reported in which one of the amide C=O bonds of the peptide backbone is replaced by an oxetane ring. They are synthesised by conjugate addition of various α-amino esters to a 3-(nitromethylene)oxetane, reduction of the nitro group and further coupling with N-Z protected amino acids to grow the peptide chain. Structural insights are provided by X-ray diffraction and molecular dynamics simulations.