Three prieurianin-type limonoids (1–3), including two new compounds (1 and 2) and one known compound (3) were isolated from Munronia henryi. These compounds were tested for their activities against tobacco mosaic virus (TMV) by the conventional half-leaf method and the outcomes were analyzed by western blotting and RT-PCR assays. The three tested compounds, at 100 μg/mL, showed strong antiviral activities in the pretreated tobacco plants with inhibition rates ranging from 70.5% to 81.3%, which were significantly higher than that of the positive control, ningnanmycin (55.6%). Their potential of inducing systemic acquired resistance (SAR) was also evaluated, in which compound 1 showed excellent induction activities. Furthermore, it was found that potentiation of defense-related enzyme activity and the contents of SA was increased. Compound 1 could also inhibit the expression of TMV CP and up-regulate the expression of defense-related genes. This work revealed that these limonoids, especially compound 1 could induce resistance in tobacco plants against the viral pathogen TMV. Meanwhile, compounds 1–3 could down-regulate the expression of NtHsp70–1 and Nthsp70–261 genes, indicating that these limonoids possibly inhibit TMV infection by suppressing NtHsp70–1 and Nthsp70–261 expression. This study is the first to report antiviral compounds with two different mechanisms of action.