A ditopic DTPA monoamide derivative containing an 8-hydroxyquinoline moiety was synthesized and the corresponding gadolinium(III) complex ([Gd(H5)(H2O)]−) was prepared. After adding aluminum(III), the 8-hydroxyquinoline part self-assembled into a heteropolymetallic triscomplex [(Gd5)3Al(H2O)3]3−. The magnetic and optical properties of this metallostar compound were investigated in order to classify it as a potential in vitro bimodal contrast agent. The proton nuclear magnetic relaxation dispersion measurements indicated that the relaxivity r1 of [Gd(H5)(H2O)]− and [(Gd5)3Al(H2O)3]3− at 20 MHz and 310 K equaled 6.17 s−1 mM−1 and 10.9 s−1 mM−1 per Gd(III) ion respectively. This corresponds to a relaxivity value of 32.7 s−1 mM−1 for the supramolecular complex containing three Gd(III) ions. The high relaxivity value is prominently caused by an increase of the rotational tumbling time τR by a factor of 2.7 and 5.5 respectively, in comparison with the commercially used MRI contrast agent Gd(III)–DTPA (Magnevist®). Furthermore, upon UV irradiation, [(Gd5)3Al(H2O)3]3− exposes green broad-band emission with a maximum at 543 nm. Regarding the high relaxivity and the photophysical properties of the [(Gd5)3Al(H2O)3]3− metallostar compound, it can be considered as a lead compound for in vitro bimodal applications.