Replacing animal-derived proteins with plant-based proteins has environmental and health benefits. Despite increasing consumer acceptance of plant-based proteins, most individuals do not frequently consume them. Understanding how different consumer groups perceive and categorise plant-based proteins in comparison to animal-derived proteins can support the protein transition, as it might provide insights into both consumer acceptance and how to position plant-based proteins. Based on categorisation theory, we investigated how different consumers – omnivores, flexitarians, vegetarians and vegans – categorise various sources of proteins and to what extent they use taxonomic, goal-derived or cross-categorisation approaches. 121 Dutch participants free-sorted 80 product cards (30 plant-based proteins, 20 animal-derived proteins, 5 hybrids (animal-plant) and 25 non-protein products). Forty participants elaborated on their categorisations in follow-up interviews. Our findings show that similar strategies can result in different categories depending on consumer group. Taxonomic categorisation strategies are dominantly applied by all consumer groups, but specific categories differ. With decreasing animal protein consumption, omnivores, flexitarians, vegetarians and vegans become increasingly strict in their categorisations. Omnivores do not separate proteins as strictly as flexitarians, vegetarians and vegans. All groups separate animal-derived meat from plant-based meat alternatives, but hybrid meat is ambiguous for omnivores and flexitarians. Variations in categorisations of plant-based proteins between groups give directions to marketers on how to tailor positioning of these products in a way that consumers identify and adopt plant-based proteins, to guide and accelerate the protein transition.