Reactive oxygen species (ROS) are commonly produced by algal, vascular plant, and animal cells involved in the innate immune response as cellular signals promoting defense and healing and/or as a direct defense against invading pathogens. The production of reactive species in macroalgae upon injury, however, is largely uncharacterized. In this study, we surveyed 13 species of macroalgae from the Western Antarctic Peninsula and show that the release of strong oxidants is common after macroalgal wounding. Most species released strong oxidants within 1 min of wounding and/or showed cellular accumulation of strong oxidants over an hour post-wounding. Exogenous catalase was used to show that hydrogen peroxide was a component of immediate oxidant release in one of five species, but was not responsible for the entire oxidative wound response as is common in vascular plants. The other component(s) of the oxidant cocktail released upon wounding are unknown. We were unable to detect protein nitration in extracts of four oxidant-producing species flash frozen 30 s after wounding, but a role for reactive nitrogen species such as peroxynitrite cannot be completely ruled out. Two species showed evidence for the production of a catalase-activated oxidant, a mechanism previously known only from the laboratory and from the synthetic drug isoniazid used to kill the human pathogen Mycobacterium tuberculosis. The rhodophyte Palmaria decipiens, which released strong oxidants after wounding, also produced strong oxidants upon grazing by a sympatric amphipod, suggesting that oxidants are involved in the response to grazing. © 2013 Phycological Society of America.