Nanocrystalline (nc) Ni films show pronounced grain growth and suffer from concomitant deterioration of their mechanical and magnetic properties after annealing at relatively low temperatures (T(ANN) ≥ 475 K). This constitutes a drawback for their applicability as coatings or in components of miniaturized devices. This work reveals that the thermal stability of nc Ni is significantly improved by controllably alloying Ni with Cu, by means of electrodeposition, to form a Ni(1-x)Cu(x) solid solution. To tune the composition of such nc alloys, Ni(1-x)Cu(x) films are deposited galvanostatically using an electrolytic bath containing Ni and Cu sulfate salts as electroactive species, saccharine as grain-refining agent, and applying current densities ranging from -10 to -40 mA cm(-2). The enhanced thermal stability is ascribed to segregation of a Cu-rich phase at the Ni(1-x)Cu(x) grain boundaries, which acts as a shielding layer against grain growth. As a result, high values of hardness (in excess of 6 GPa) remain in nc Ni(1-x)Cu(x) for x ≥ 0.3, even after annealing at T(ANN) ≥ 575 K. From a magnetic point of view, Ni(1-x)Cu(x) films possess lower coercivity values than pure nc Ni films, both in the as-prepared and annealed states, thus offering potential advantages for certain soft magnetic applications.