Three new syncarpic acid-conjugated sesquiterpenoids, tomentodiones E-G (1-3), and six new syncarpic acid-conjugated monoterpenoids, tomentodiones H-M (4-9), were isolated from the leaves of Rhodomyrtus tomentosa. Compounds 1-3 represent the first examples of β-calacorene-based meroterpenoids. Their structures and absolute configurations were determined by a combination of NMR and ECD spectroscopy and X-ray diffraction analysis. On the basis of ECD data analysis for isolated and synthesized compounds, an empirical rule was proposed to determine the absolute configuration at C-7' of syncarpic acid-conjugated terpenoids. Additionally, a study of the reversal effect of multidrug resistance in doxorubicin-resistant human breast cancer cells showed that the noncytotoxic (+)-4 exerted the strongest potentiation effect of doxorubicin susceptibility, with an enhancement of 16.5-fold at a concentration of 30 μM.