Upon Salmonella infection, bacterial effectors proteins are delivered into host cells by specialised multiprotein secretion systems. They modulate a range of host cellular processes, helping prevent bacterial clearance and promoting bacterial replication and survival within the host cell. Although many Salmonella effectors have been characterised, the type III secretion system effector SteE remains largely uncharacterised in terms of structure and function. This study revealed that SteE is predominately an intrinsically disordered protein, which may mimic the disordered regions commonly found in eukaryotic signalling proteins. Interaction studies with recombinant proteins demonstrated that SteE forms a stable and direct interaction with GSK3β, one of its identified host interaction partners, and addition of ATP increased SteE/GSK3 complex formation. Complex stabilisation is highly dependent on the conserved T91 residue of SteE, which GSK3β directly phosphorylates. SteE interaction with GSK3β is also dependent on conserved sequence motifs in SteE, one of which mimics a canonical GSK3β phosphorylation site. While SteE has multiple GSK3 binding sites, SteE interacts with the canonical substrate binding site in GSK3β. The interaction of SteE with GSK3β and the sequential SteE phosphorylation by GSK3β is hypothesised to stimulate a conformational change that increases the binding affinity of SteE with GSK3β and alters the substrate amino acid specificity of GSK3β from a Ser/Thr kinase to a Tyr kinase. The subsequent phosphorylation of Y143 in SteE by GSK3β forms the ‘pYxxQ’ SH2 binding motif that enables the recruitment of STAT3 via its SH2 domain for the phosphorylation of STAT3 at Y705. Overall, this work conveyed that SteE acts as an adaptor protein, recruiting a host kinase to a non-canonical substrate, and as a functional regulator of the well-known host Ser/Thr kinase GSK3, enabling Tyr phosphorylation on substrates. This unique and distinct function of SteE was also evident in the putative SteE homologs from diverse bacteria.