Monodora tenuifolia (Benth.) W. Ash (Annonaceae) is a tree used in traditional medicine for treating diseases like dysentery, dermatitis, and headache, while its seeds are used as a condiment. Despite its multiple uses, only one chemical constituent study had been conducted prior to this work. In a continuation of investigating Nigerian medicinal plants, a prenylated indole was isolated from the stem bark of M. tenuifolia. The compound was purified via silica gel and neutral alumina chromatography, and identified as 3-dimethylallylindole (1) using spectral data (uv, ir, ¹H-nmr, ¹³C-nmr, ms) and comparison with a synthetic sample. Antimicrobial assays showed that 3-dimethylallylindole exhibited antibacterial activity against Mycobacterium smegmatis, Staphylococcus aureus, and Bacillus subtilis, and antifungal activity against yeasts (Candida albicans, Saccharomyces cerevisae, Cryptococcus neoformans) and filamentous fungi (Trichophyton mentagrophytes, Pycnoporus sanguineus, Helminthosporium sp.). The minimum inhibitory concentration (MIC) values indicated it was more effective as an antifungal agent, with MICs as low as 0.78 μg/ml against Helminthosporium sp. This represents the first report of the antimicrobial activity of 3-dimethylallylindole, which is an antibacterial and antifungal metabolite from M. tenuifolia.