NUAK1 (NUAK family SnF1-like kinase-1) and NUAK2 protein kinases are activated by the LKB1 tumour suppressor and have been implicated in regulating multiple processes such as cell survival, senescence, adhesion and polarity. In the present paper we present evidence that expression of NUAK1 is controlled by CDK (cyclin-dependent kinase), PLK (Polo kinase) and the SCF^βTrCP (Skp, Cullin and F-box^βTrCP) E3 ubiquitin ligase complex. Our data indicate that CDK phosphorylates NUAK1 at Ser⁴⁴⁵, triggering binding to PLK, which subsequently phosphorylates NUAK1 at two conserved non-catalytic serine residues (Ser⁴⁷⁶ and Ser⁴⁸⁰). This induces binding of NUAK1 to βTrCP, the substrate-recognition subunit of the SCF^βTrCP E3 ligase, resulting in NUAK1 becoming ubiquitylated and degraded. We also show that NUAK1 and PLK1 are reciprocally controlled in the cell cycle. In G₂–M-phase, when PLK1 is most active, NUAK1 levels are low and vice versa in S-phase, when PLK1 expression is low, NUAK1 is more highly expressed. Moreover, NUAK1 inhibitors (WZ4003 or HTH-01-015) suppress proliferation by reducing the population of cells in S-phase and mitosis, an effect that can be rescued by overexpression of a NUAK1 mutant in which Ser⁴⁷⁶ and Ser⁴⁸⁰ are mutated to alanine. Finally, previous work has suggested that NUAK1 phosphorylates and inhibits PP1β^MYPT1 (where PP1 is protein phosphatase 1) and that a major role for the PP1β^MYPT1 complex is to inhibit PLK1 by dephosphorylating its T-loop (Thr²¹⁰). We demonstrate that activation of NUAK1 leads to a striking increase in phosphorylation of PLK1 at Thr²¹⁰, an effect that is suppressed by NUAK1 inhibitors. Our data link NUAK1 to important cell-cycle signalling components (CDK, PLK and SCF^βTrCP) and suggest that NUAK1 plays a role in stimulating S-phase, as well as PLK1 activity via its ability to regulate the PP1β^MYPT1 phosphatase.