Two sesterterpenoids, possessing an unusual 10,11-seco-gentianellane skeleton, gentianelloids A and B, were isolated from a traditional Uighur medicine Gentianella turkestanorum. Through extensive spectroscopic analysis and single-crystal X-ray diffraction, their structures including absolute configurations were unambiguously determined. A plausible biosynthetic pathway for the two compounds was proposed. Both compounds showed remarkable immunosuppressive activity, including inhibition of the proliferation, activation, and cytokine IFN-γ production of T cells. The findings suggested that sesterterpenoids could contribute positively to the therapeutic effects of this popular traditional Uighur medicine.