Updated project metadata. Cytokines are highly pleiotropic ligands that critically contribute to a balanced immune response. We have an incomplete understanding of how cytokines elicit their functional pleiotropy, which has limited their therapeutic efficacy. Here, using Interleukin-6 (IL-6) as a model system, we have performed detailed phosphoproteomic and transcriptomic studies in human CD4 + T helper (Th)-1 cells to address the molecular bases defining cytokine functional pleiotropy. We have identified cyclin dependent kinase (CDK)8 as a new negative regulator of transcriptional activities of signal transducer and activator of transcription (STAT)3. We found that CDK8 is a major regulator of the IL-6 phosphoproteome and interacts with STAT3 in the nucleus upon IL-6 stimulation. Inhibition of CDK8 activity, using specific small molecules inhibitors, reduced the IL-6-induced phosphoproteome by 23% in Th-1 cells, including STAT3 S727 phosphorylation. STAT3 binding to target DNA sites in the genome was increased upon CDK8 inhibition, which resulted in a concomitant increase in STAT3 mediated transcriptional activity. Importantly, inhibition of CDK8 activity under Th-17 polarizing conditions resulted in an enhancement of Th-17 differentiation. Our results support a model where CDK8 regulates STAT3 transcriptional processivity via modulation of its gene loci resident time, critically contributing to diversification of IL-6 responses.