The phosphine-mediated [3 + 3] annulations of aziridines and allenes are experimentally simple reactions, run under very mild conditions, for the preparation of highly functionalized tetrahydropyridines in yields of up to 98% and trans/cis ratios of up to 97:3. In addition to steps that are typical of nucleophilic phosphine-catalyzed reactions of allenoates, the mechanism of this new reaction features apparent nucleophilic aromatic substitution and concomitant desulfonylation, processes hitherto unknown during phosphine-promoted cycloaddition reactions. Notably, these reactions are the first reported examples of aziridines as reaction partners in nucleophilic phosphine-catalyzed transformations.