Mutualistic nutritional symbioses constitute a widespread phenomenon in marine ecosystems and are frequently defined as relationships in which all partners mutually benefit. In New Zealand, the sea anemone Handactis nutrix (Stuckey, 1909) lives in ectosymbiosis with different brown macroalgae such as species of Carpophyllum and Cystophora. H. nutrix is not reported to occur without the seaweed, suggesting that there may be benefits for both partners. This study, undertaken around Wellington’s south coast, found compelling evidence for a nutritional symbiosis between the sea anemone Handactis nutrix and one of its seaweed hosts, Cystophora scalaris, using stable isotope analysis to trace carbon and nitrogen transfer between the symbiotic partners. While no transfer of carbon could be detected between the partners, there was strong evidence for the transfer of nitrogen in the form of ammonium from the sea anemone to the seaweed. Additionally, the natural abundance of the nitrogen isotope 15N differed between seaweeds associated with H. nutrix and those without, further supporting the notion of a nutritional symbiosis. These findings, coupled with the observed host specificity towards species of Cystophora compared to Carpophyllum and specific assemblage patterns of the sea anemone, strongly suggest a mutually beneficial relationship. This nutritional symbiosis therefore likely has significant implications for the survival, growth, and overall health of both the sea anemone and its seaweed host.