Cashew testa (Anacardium occidentale L.) is an underutilized by-product rich in bioactive compounds. This study aims to evaluate the polyphenol content and bioactivities of cashew nut testa in Vietnam using ultrasound-assisted extraction with different solvents. Methanol and ethanol were identified as the most effective solvents, yielding extracts with high polyphenol content of (484.06 ± 4.26) mg GAE/g and (465.33 ± 4.69) mg GAE/g, respectively. The results from antioxidants assay demonstrated that at the concentration of 100 µg/mL, methanol and ethanol extracts exhibited > 87% DPPH and > 99% ABTS inhibition In addition, both extracts showed pronounced antimicrobial activity against Staphylococcus aureus and Candida albicans, as indicated by inhibition zone diameters exceeding 11 mm and minimum inhibitory concentrations of 1.56 mg/mL. These results confirm the effectiveness of ultrasound-assisted extraction for producing polyphenol-rich extracts with antioxidant and antimicrobial potentials.