Phytochemical investigation of the semi-parasitic plant, Plicosepalus curviflorus (Loranthaceae) growing in Saudi Arabia resulted in the isolation of a new catechin–gallic acid derivative of inositol, plicosepalin A ( 1 ) [(+) catechin-4′- O -(1″- O -galloyl-5″- O -methyl)- myo -inositol], along with seven known compounds: methyl gallate ( 2 ), catechin ( 3 ), quercetin ( 4 ), gallic acid ( 5 ), lupeol ( 6 ), β-sitosterol ( 7 ), and ursolic acid ( 8 ). Their structures were elucidated on the basis of spectroscopic analyses, including HRESIMS, ESIMS, 1 H and 13 C NMR, HSQC, and HMBC, as well as comparison with reported data. The antioxidant and antimicrobial activities of 1 were evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and the disc diffusion assay, respectively. Compound 1 exhibited potent free radical scavenging activity with an IC 50 value of 9.0 ± 0.27 μM. Moreover, significant activities against Staphylococcus aureus and Bacillus subtilis were recorded.