Popcorn is considered gluten-free. The kernels contain corn gluten, which is not the type of gluten that causes reactions in people with gluten sensitivities. Toppings such as movie-theater butter do not typically contain gluten, but care should be taken to evaluate all flavorings for gluten.
Followers of gluten-free diets avoid wheat, barley and rye. These grains are especially harmful to those with celiac disease. Gluten causes damage to the intestinal wall of individuals with celiac disease- even in small amounts.
Adherents to gluten-free diets should read the labels on popcorn closely or ask for nutritional information if dining out. Gluten can be found in small amounts in seemingly unlikely processed foods.