Protein-energy malnutrition (PEM) is a growing concern on account of an aging population and its negative health consequences. While dietary protein plays a key role in the prevention of PEM, it also plays a pivotal role in the environmental impact of the human diet. In search for sustainable dietary strategies to increase protein intake in older adults, this study investigated the readiness of older adults to accept the consumption of the following alternative, more sustainable protein sources: plant-based protein, insects, single-cell protein, and in vitro meat. Using ordinal logistic regression modeling, the associations of different food-related attitudes and behavior and sociodemographics with older adults' acceptance to consume such protein sources were assessed. Results were obtained through a consumer survey among 1825 community-dwelling older adults aged 65 years or above in five EU countries (United Kingdom, the Netherlands, Poland, Spain, and Finland). Dairy-based protein was generally the most accepted protein source in food products (75% of the respondents found its consumption acceptable or very acceptable). Plant-based protein was the most accepted alternative, more sustainable protein source (58%) followed by single-cell protein (20%), insect-based protein (9%), and in vitro meat-based protein (6%). We found that food fussiness is a barrier to acceptance, whereas green eating behavior and higher educational attainment are facilitators to older adults' acceptance to eat protein from alternative, more sustainable sources. Health, sensory appeal, and price as food choice motives, as well as gender and country of residence were found to influence acceptance, although not consistently across all the protein sources. Findings suggest that there is a window of opportunity to increase older adults' acceptance of alternative, more sustainable protein sources and in turn increase protein intake in an environmentally sustainable way in EU older adults.