In this study, tris(4-carboxy-3-benzyl-3-azabutyl) amine—a novel polyaminocarboxylate ligand—was synthesized. Two new nonionic complexes (a mononuclear complex and a dinuclear complex) of gadolinium with this ligand were also synthesized as potential magnetic resonance imaging (MRI) contrast agents. The structure of the dinuclear complex was determined by X-ray crystal structure analysis. The relaxivity and cytotoxicity of the mono- and dinuclear complexes were evaluated.