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A phytochemical investigation on the roots of medicinal plant <i>Eurycoma longifolia</i> resulted in the isolation of 10 new highly oxygenated C₂₀ quassinoids longifolactones G‒P (<b>1</b>–<b>10</b>), along with four known ones (<b>11</b>–<b>14</b>). Their chemical structures and absolute configurations were unambiguously elucidated on the basis of comprehensive spectroscopic analysis and X-ray crystallographic data. Notably, compound <b>1</b> is a rare pentacyclic C₂₀ quassinoid featuring a densely functionalized 2,5-dioxatricyclo[5.2.2.0^4,8]undecane core. Compound <b>4</b> represents the first example of quassinoids containing a 14,15-epoxy functionality, and <b>7</b> features an unusual α-oriented hydroxyl group at C-14. All isolated compounds were evaluated for their anti-proliferation activities on human leukemia cells. Among the isolates, compounds <b>5</b>, <b>12</b>, <b>13</b>, and <b>14</b> potently inhibited the in vitro proliferation of K562 and HL-60 cells with IC₅₀ values ranging from 2.90 to 8.20 μM.