Bioactivity-guided fractionation of an antimicrobial active extract from twigs of Baccharis retusa C. DC. (Asteraceae) yielded the flavanone 5,4'-dihydroxy-7-methoxy-flavanone (sakuranetin) as responsible for the detected activity. The structure of the bioactive compound was established on the basis of spectroscopic data analysis, including NMR and MS. Additionally, the structure of a new crystal form of sakuranetin was confirmed by X-ray diffratometry. The minimum inhibitory concentrations (MIC) of isolated compound were determined against pathogenic yeast belonging to the genus Candida (six species), Cryptococcus (two species/four serotypes) and S. cerevisiae BY 4742 (S288c background) and ranged from 0.32 to 0.63 μg/μL. Our results showed that sakuranetin, which structure was fully characterized, could be used as a tool for the design of novel and more efficacious antifungal agents.