Magnetic molecules are potential building blocks for the design of spintronic devices(1,2). Moreover, molecular materials enable the combination of bottom-up processing techniques, for example with conventional top-down nanofabrication(3). The development of solid-state spintronic devices based on the giant magnetoresistance(4), tunnel magnetoresistance(5) and spin-valve effects(6) has revolutionized magnetic memory applications. Recently, a significant improvement of the spin-relaxation time has been observed in organic semiconductor tunnel junctions(7,8), single non-magnetic molecules coupled to magnetic electrodes have shown giant magnetoresistance(9,10) and hybrid devices exploiting the quantum tunnelling properties of single-molecule magnets have been proposed(2). Herein, we present an original spin-valve device in which a nonmagnetic molecular quantum dot, made of a single-walled carbon nanotube contacted with non-magnetic electrodes, is laterally coupled through supramolecular interactions to TbPc(2) single-molecule magnets (Pc = phthalocyanine). Their localized magnetic moments lead to a magnetic field dependence of the electrical transport through the single-walled carbon nanotube, resulting in magnetoresistance ratios up to 300% at temperatures less than 1 K. We thus demonstrate the functionality of a supramolecular spin valve without magnetic leads. Our results open up prospects of new spintronic devices with quantum properties.