An anti-HIV screening of natural product extracts resulted in the discovery of a new antiviral protein through bioassay-guided fractionation of an aqueous extract of the ascidian Didemnum molle. The protein was sequenced through a combination of tandem mass spectroscopy and N-terminal Edman degradation of peptide fragments after a series of endoproteinase digestions. The primary amino acid sequence and disulfide bonding pattern of the 102-amino acid protein were closely related to the antiviral protein cyanovirin-N (CV-N). This new CV-N homolog was named Dm-CVNH. Alphafold2 prediction resulted in a tertiary structure, highly similar to CV-N, comprised of two symmetrically related domains that contained five β-strands and two α-helical turns each. Dm-CVNH showed specificity for high mannose and oligomannose structures, bound to HIV-1 gp-120 and potently inactivated HIV in neutralization assays (EC