We have grown a series of nickel-substituted single crystals of the layered ferromagnet (FM) Fe3GeTe2. The large single-crystalline samples of (Fe1−xNix)3GeTe2 with x=0–0.84 were characterized with single-crystal x-ray diffraction, magnetic susceptibility, electrical resistance, and muon spin spectroscopy. We find Fe can be continuously substituted with Ni with only a minor structural variation. In addition, FM order is suppressed from TC=212K for x=0 down to TC=50Kfor x=0.3, which is accompanied by a strong suppression of saturated and effective moments, and Curie-Weiss temperature. Beyond x=0.3, the FM order is continuously smeared into a FM cluster-glass phase, with a nearly full magnetic volume fraction. We attribute the observed change in the nature of magnetic order to the intrinsically disordered structure of Fe3GeTe2 and subsequent dilution effects from the Ni substitution.