Given the rapid expansion of aquaculture and the need to identify new ingredients for formulating diets for aquatic organisms, pumpkin (Cucurbita sp.) by-products—such as seeds, peels, and pulp—emerge as an economical and sustainable option. Therefore, this review aimed to compile information on the use of pumpkin by-products in aquaculture, focusing on the nutrition of fish and crustaceans. A data survey was conducted with emphasis on the nutritional composition, amino acid profile, and fatty acid profile of the most commonly cultivated pumpkin species in Brazil and worldwide. Additionally, the main bioactive compounds found in pumpkin by-products and their respective nutraceutical roles in the organism are presented, along with antinutritional factors and key processing methods of these by-products.