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Flowers of <i>Inula britannica</i> commercially serve as pharmaceutical herbs in the manufacturing of medicinal products. In the current study, sesquiterpenoids of <i>I. britannica</i> flowers’ extract and their potential effects against triple-negative breast cancer (TNBC) cells were investigated. Eight structurally diverse sesquiterpenoids, including one sesquiterpenoid dimer (<b>1</b>) and seven sesquiterpenoid monomers (<b>2</b>–<b>8</b>) were isolated from this source. The structures of all compounds were elucidated by 1D/2D NMR data, and their absolute configurations were discerned by single crystal X-ray diffraction. All of the compounds were tested for their potential effects against TNBC. Specifically, <b>5</b> displayed strong antiproliferative potency against TNBC cells with a high selective index (SI) on MCF-7 cells (SI > 4 of IC₅₀ on MDA-MB-468/IC₅₀ on MCF-7), and dimer <b>1</b> (IC₅₀ = 8.82 ± 0.85 μM) showed better antiproliferative potency against MCF-7 cells than the other monomers did (<b>2</b>–<b>8</b>) (IC₅₀ > 20 μM). To our best knowledge, compound <b>5</b> is the first sesquiterpenoid targeting TNBC cells.