Salmonella causes ∼1 million cases of gastroenteritis annually in the United States. Critical to virulence are secreted effectors that reprogram host functions. We previously discovered the effector SarA facilitates phosphorylation of STAT3, inducing expression of the anti-inflammatory cytokine interleukin-10 (IL-10). This STAT3 activation requires a region of homology with the host cytokine receptor gp130. Here, we demonstrate that a single amino acid difference is critical for the anti-inflammatory bias of SarA-STAT3 signaling. An isoleucine at pY+1 of the YxxQ motif in SarA (which binds the STAT3 SH2 domain) causes increased STAT3 recruitment and phosphorylation, biasing toward anti-inflammatory targets. This isoleucine renders SarA a better substrate for tyrosine phosphorylation by GSK-3. GSK-3 is canonically a serine/threonine kinase that nonetheless undergoes tyrosine autophosphorylation at a motif with isoleucine at the pY+1 position. Our results provide a molecular basis for how a Salmonella effector achieves supraphysiological levels of STAT3 activation to control host genes.