Monodora tenuifolia (Benth.) W. Ash (Annonaceae) is a tree found in evergreen and transition forests along the west coast of Africa, with its morphological parts used in traditional medicine for treating dysentery, dermatitis, headache, toothache, and as a vermifuge, and aromatic seeds as a condiment. Despite its numerous uses, only one investigation on its chemical constituents has been carried out. In a continuation of the investigation of Nigerian medicinal plants, a prenylated indole was isolated from the stem bark of M. tenuifolia. This paper reports the isolation, identification, and antimicrobial activity of this prenylated indole. The stem bark was extracted with EtOAc, and the resulting red oily residue was chromatographed consecutively over silica gel 60 and neutral alumina to obtain compound 1 (mp 45-47°C). Based on UV (absorption bands at λ max 291, 282, and 225 nm characteristic of an indole chromophore), IR (amino group band at 3400 cm⁻¹), ¹H-NMR (aromatic protons of indole, olefinic proton as a triplet, 2H doublet, and 6H singlet for =C(CH₃)₂), ¹³C-NMR (comparison with indole, 2-methylindole, and 3-methylindole confirming C-3 substitution), and MS (parent ion at m/z 185 and fragments corresponding to M⁺-2CH₃ and M⁺-C₅H₈) data, as well as identity with a synthetic sample, compound 1 was identified as 3-dimethylallylindole. Antimicrobial assays showed that 3-dimethylallylindole exhibits antibacterial activity against Mycobacterium smegmatis, Staphylococcus aureus, and Bacillus subtilis, and antifungal activity against Candida albicans, Cryptococcus neoformans, Saccharomyces cerevisiae, Trichophyton mentagrophytes, Pycnoporus sanguineus, and Helminthosporium sp. Minimum inhibitory concentration (MIC) values indicated it is more effective as an antifungal agent, with MICs of 1.56 μg/ml against T. mentagrophytes and 0.78 μg/ml against Helminthosporium sp. This represents the first report of the antimicrobial activity of 3-dimethylallylindole.