Magnetic dilution of a well-established Kitaev candidate system is realized in the substitutional Ru 1−x Rh x Cl 3 series (x = 0.02-0.6). Optimized syntheses protocols yield uniformly doped single crystals and polycrystalline powders that are isostructural to the parental α-RuCl 3 as per x-ray diffraction. The Rh content x is accurately determined by the quantitative energy-dispersive x-ray spectroscopy technique with standards. We determine the magnetic phase diagram of Ru 1−x Rh x Cl 3 for in-plane magnetic fields from magnetization and specific-heat measurements as a function of x and stacking periodicity and identify the suppression of the magnetic order at x ≈ 0.2 towards a disordered phase, which does not show any clear signature of freezing into a spin glass. Comparing with previous studies on the substitution series Ru 1−x Ir x Cl 3 , we propose that chemical pressure would contribute to the suppression of magnetic order, especially in Ru 1−x Ir x Cl 3 , and that the zigzag magnetic ground state appears to be relatively robust with respect to the dilution of the Kitaev--Heisenberg magnetic lattice. We also discovered a slight dependence of the magnetic properties on thermal cycling, which would be due to an incomplete structural transition.