We describe a new technique in protein synthesis that extends the existing repertoire of methods for protein modification: A chemoselective reaction that induces reactivity for a subsequent bioconjugation. Hereby, an azide building block reacts first with an ethynylphosphonite via the Staudinger‐phosphonite reaction (SPhR) to an ethynylphosphonamidate. The resulting electron‐deficient triple bond subsequently undergoes a cysteine‐selective reaction with proteins or antibodies. We demonstrate that ethynylphosphonamidates hold excellent cysteine‐selective reactivity combined with a superior stability of the thiol adducts. This turns our technique into a versatile and powerful tool for the facile construction of stable, functional protein conjugates.