<p>Appropriate T-cell functional polarization is critical for maintaining immune stability and immune tolerance. The role of <i>Fam234a</i> in the functional polarization of T cells is unknown. In a DSS-induced inflammatory bowel disease model in Rag2^-/- mice with either naive WT or <i>Fam234a</i>-deficient CD4⁺ T cells, mice with <i>Fam234a</i>-deficient CD4⁺ T cells presented milder symptoms of colitis, accompanied by a decreased ratio of Th17/Treg cells. Consistent with the in vivo observations, Th17 differentiation was significantly decreased and Treg induction was increased in the in vitro naive <i>Fam234a</i>-deficient CD4⁺ T-cell polarizing induction system. Similarly, knocking down <i>FAM234A</i> in human T cells using siRNA also revealed that FAM234A deficiency significantly decreased the Th17/Treg cell ratio in human T cells. Coimmunoprecipitation-mass spectrometry (Co-IP-MS), protein interaction, and biochemical studies revealed that FAM234A may directly interact with the deubiquitinase USP4 to affect its deubiquitination function. The reduction in Th17 cells and increase in Treg cells among <i>Fam234a</i>-deficient T cells were significantly reversed by restoring USP4 overexpression. RNA sequencing and molecular studies indicated that <i>Fam234a</i> knockout reduced USP4-mediated Rheb and RORγt deubiquitination, mTOR activation, and <i>Hif1α</i> expression and ultimately affected Th17 and Treg differentiation. Therefore, <i>Fam234a</i> intrinsically balances the Th17 and Treg differentiation of naive CD4⁺ T cells by directly preventing USP4-mediated deubiquitination of Rheb to regulate mTOR-HIF1α-related oxidative phosphorylation and glycolytic gluconeogenesis metabolism pathways as well as USP4-mediated deubiquitination of RORγt pathways. This research revealed the critical role of FAM234A in the orchestration of Th17/Treg cell fate decisions and may offer potential therapies for their related diseases.</p>