Abstract: Vanadium-dependent haloperoxidases (vHPOs) are essential enzymes in macroalgae, known for their ability to catalyze halogenation reactions using halides and hydrogen peroxide. These enzymes facilitate the biosynthesis of halogenated compounds that contribute to the defense mechanisms of algae and are considered to play a crucial role in the oxidative stress response. Using a transcriptomic approach, we discovered and analyzed about 70 novel vHPO sequences from nine different macroalgae species collected in the Baltic Sea. All sequences were carefully selected for their universal catalytic center characterized by structurally conserved residues essential for catalysis and vanadate-binding. Mass spectrometry provided evidence for eight of those proteins in extract fractions of the brown alga Saccharina latissima. Using RT-qPCR, we also investigated the role of vHPO gene expression in stress response on a subset of S. latissima vHPO transcripts by exposing it to different stressors including copper excess, oxidative stress, and biotic stress mimicked by elicitor treatment using a S. latissima tissue homogenate. The optimal quantum yield of photosystem ll served as a physiological plant stress parameter (F_V/F_M). Our data support the hypothesis that these enzymes are involved in mitigating oxidative stress, as suggested by up-regulation in gene expression following H₂O₂ and elicitor treatments, and are likely involved in environmental stress response. This work advances our understanding of vHPO function in marine algae and highlights their potential role in responding to environmental stress.