Five new hydantoin alkaloids, named parazoanthines A-E (1-5), were isolated as the major constituents of the Mediterranean sea anemone Parazoanthus axinellae. Their structural elucidation was achieved through NMR spectroscopic and mass spectrometric analyses. The absolute configuration of the chiral compounds 1 and 4 was determined by comparison between experimental and TDDFT-calculated CD spectra. The configuration of the trisubstituted double bond of 2, 3, and 5 was deduced from the (3)J(H6-C4) coupling constant value. This family of alkaloids represents the first example of natural 3,5-disubstituted hydantoins that do not exhibit a methyl at N-3. All compounds were tested for their natural toxicity (Microtox assay), and parazoanthine C (3) exhibited the highest natural toxicity.