The red knot (Calidris canutus) is a migratory shorebird experiencing a climate-driven trophic mismatch during breeding, resulting in smaller chicks that develop into juveniles with shorter bills. These birds are unable to reach their preferred prey and have adapted by shifting from a deeper bivalve- to shallower seagrass-dominated diet. The nutritional adequacy of diets that lack animal protein, however, remains unclear. We conducted a controlled diet-change experiment in the laboratory with two groups of five captive red knots fed a standard diet, rich in animal protein for 13 weeks, followed by 42 days on a strictly vegetarian diet. Body mass and faecal matter were monitored as the main indicators of overall health. Body mass initially declined after the diet change but then gradually increased and stabilized to levels normal for the season and stage. Faecal matter transitioned from green to dry brown, and then to a colour and consistency similar to that of wild red knots, but notably in a higher amount. Upon returning to the standard, high-protein diet, the body mass of birds rapidly increased, was abnormally high for two days, and then stabilized to normal levels. This study is one of the first controlled laboratory experiments to examine a herbivorous diet in carnivorous shorebirds and demonstrated that red knots can adapt to and survive on a strictly plant-based diet. Future measurements will characterize potential changes in the gut microbiome of red knots in captivity in relation to these diet shifts.