Little is known about the impact of n3 (ω3) PUFAs on polarization of CD4(+) T cells into effector subsets other than Th1 and Th2. We assessed the effects of dietary fat [corn oil (CO) vs. fish oil (FO)] and fermentable fiber [cellulose (C) vs. pectin (P)] (2 × 2 design) in male C57BL/6 mice fed CO-C, CO-P, FO-C, or FO-P diets for 3 wk on the ex vivo polarization of purified splenic CD4(+) T cells (using magnetic microbeads) into regulatory T cells [Tregs; forkhead box P3 (Foxp3(+)) cells] or Th17 cells [interleukin (IL)-17A(+) and retinoic acid receptor-related orphan receptor (ROR) γτ(+) cells] by flow cytometry. Treg polarization was unaffected by diet; however, FO independently reduced the percentage of both CD4(+) IL-17A(+) (P < 0.05) and CD4(+) RORγτ(+) cells (P < 0.05). Moreover, expression of another critical Th17-cell-related transcription factor, signal transducer and activator of transcription 3, was reduced by FO. Dietary FO reduced the surface expression of both IL-6R and IL-23R on polarized Th17 cells (P ≤ 0.05), thus interfering with the promotive effects of these critical cytokines on Th17 polarization. Additionally, C57BL/6 mice fed diets enriched in eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), or DHA + EPA similarly reduced Th17-cell polarization in comparison to CO by reducing expression of the Th17-cell signature cytokine (IL-17A; P = 0.0015) and transcription factor (RORγτ P = 0.02), whereas Treg polarization was unaffected. Collectively, these data show that n3 PUFAs exert a direct effect on the development of Th17 cells in healthy mice, implicating a novel n3 PUFA-dependent, anti-inflammatory mechanism of action via the suppression of the initial development of this inflammatory T-cell subset.