Background and aims: Research in malnutrition in older people is limited by the lack of consensus on relevant outcomes. Researchers of two European initiatives, the ‘Malnutrition in the Elderly (MaNuEL) Knowledge Hub’ (mostly experts in nutrition) and the Optimal Evidence-Based Non-drug Therapies in Older People (ONTOP) project (geriatricians) agreed to merge forces performing a systematic review of the effectiveness of nutritional interventions for the prevention and treatment of malnutrition in older persons. In a first step, we aimed to identify relevant outcomes for this review using a systematic approach and to explore if the rating of relevant outcomes differed depending on the researchers’ professional background. Methods: Following the ONTOP protocol, we searched all outcomes used in research of nutritional interventions for the prevention and treatment of malnutrition in older people. We carried out a web-based Delphi survey using a standardized list of 13 potentially relevant outcomes among 41 experts in geriatrics and nutrition who were asked to rate each outcome from 1 to 9 points: low importance (score 1–3), important but non-critical (score 4–6), and critical (score 7–9). Participants were informed that only those outcomes rated as critical (7–9 points) would be used in the literature review. After two rounds consultation, we compared the results from each group of experts: the ONTOP group formed by 13 geriatricians and the MaNuEL group formed by 28 experts in nutrition. Mean values were used for overall rating and the Mann–Whitney U test was used to see the differences on ratings between both groups. Results: Mortality, morbidity, functional status, nutritional status and quality of life were considered critical outcomes by the whole group of experts. However, by analysing the ratings by the experts’ professional background, geriatricians only rated mortality, morbidity and functional status as critical, while experts in nutrition (MaNuEL group) rated nutritional status, changes in dietary intake, compliance with the intervention, quality of life, and frailty status outcomes as critical too. Two outcomes, changes in dietary intake and compliance with the intervention, were rated with a significant different between the two professional groups (p < 0.05). Conclusions: Five outcomes were considered critical for research in nutritional interventions for the prevention and treatment of malnutrition in older persons: mortality, morbidity, functional status, nutritional status and quality of life by the whole panel of experts. However, more consensus is needed on the relevance of specific outcomes of nutritional interventions. Consensus processes within but also between relevant organizations are required to reach consensus and to contribute to this aim.