The nickel-catalyzed synthesis of tetrasubstituted cyclobutenes from alkynes is reported. This transformation is uniquely promoted by the use of a primary aminophosphine, an unusual ligand in nickel catalysis. Mechanistic insights for this new transformation are provided, and post-reaction modifications of the cyclobutene products to stereodefined cyclic and acyclic compounds are reported, including the synthesis of epi-truxillic acid.