Five new decalin derivatives (1-5), together with two known compounds (6 and 7), were isolated from the ethyl acetate extract of red yeast rice. Their structures were elucidated by means of NMR and mass spectroscopic analyses. Monascusic lactone A (1) is the first reported naturally occurring decalin derivative possessing a spiro lactone at the C-1 position. The immunosuppressive effects of all these isolates (1-7) on human T cell proliferation were investigated, and all, especially monascusic acids B (2), C (3), D (4), and A (6) and heptaketide (7), suppressed human T cell proliferation in a dose-dependent manner from 10 to 100 μM. This is the first report on the immunosuppressive activity of decalin derivatives.