Abstract: In continuation of research aimed at identifying anti-inflammatory agents from natural sesquiterpenoids, an activity-guided fractionation approach utilizing lipopolysaccharide (LPS)-mediated RAW264.7 cells was employed to investigate chemical constituents from Inula Britannica (I. britannica). Seven novel sesquiterpenoid dimers inulabritanoids A−G (1−7) and two novel sesquiterpenoid monomers inulabritanoids H (8) and I (9) were isolated from I. britannica together with eighteen known compounds (10−27). The structural elucidation was accomplished through comprehensive analysis of 1D and 2D nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HR-MS), and electronic circular dichroism (ECD) spectra, complemented by quantum chemical calculations. Compounds 1, 2, 12, 16, 19, and 26 demonstrated inhibitory effects on NO production, with IC₅₀ values of 3.65, 5.48, 3.29, 6.91, 3.12, and 5.67 μmol·L⁻¹, respectively. Mechanistic studies revealed that compound 1 inhibited IκB kinase β (IKKβ) phosphorylation, thereby blocking nuclear factor κB (NF-κB) nuclear translocation, and activated the kelch-like ECH-associated protein 1 (Keap1)/nuclear factor erythroid 2-related factor 2 (Nrf2) signal pathway, leading to decreased expression of NADPH oxidase 2 (NOX-2), inducible nitric oxide synthase (iNOS), tumor necrosis factor α (TNF-α), interleukin-6 (IL-6), monocyte chemotactic protein-1 (MCP-1), IL-1β, and IL-1α and increased expression of NAD(P)H: quinone oxidoreductase 1 (NQO-1) and heme oxygenase-1 (HO-1), thus exhibiting anti-inflammatory effects in vitro. These results indicate that dimeric sesquiterpenoids may serve as promising candidates for anti-inflammatory drug development.