Using co‐immunoprecipitation combined with MS analysis, we identified the α′ subunit of casein kinase 2 (CK2) as an interaction partner of the atypical Rio1 protein kinase in yeast. Co‐purification of Rio1p with CK2 from Δcka1 or Δcka2 mutant extracts shows that Rio1p preferentially interacts with Cka2p in vitro. The C‐terminal domain of Rio1p is essential and sufficient for this interaction. Six C‐terminally located clustered serines were identified as the only CK2 sites present in Rio1p. Replacement of all six serine residues by aspartate, mimicking constitutive phosphorylation, stimulates Rio1p kinase activity about twofold in vitro compared with wild‐type or the corresponding (S > A)6 mutant proteins. Both mutant alleles (S > A)6 or (S > D)6 complement in vivo, however, growth of the RIO1 (S > A)6 mutant is greatly retarded and shows a cell‐cycle phenotype, whereas the behaviour of the RIO1 (S > D)6 mutant is indistinguishable from wild‐type. This suggests that phosphorylation by protein kinase CK2 leads to moderate activation of Rio1p in vivo and promotes cell proliferation. Physiological studies indicate that phosphorylation by CK2 renders the Rio1 protein kinase susceptible to proteolytic degradation at the G1/S transition in the cell‐division cycle, whereas the non‐phosphorylated version is resistant.