Proximate composition, amino acid and fatty acid profiles, and mineral and vitamin contents of by-products from Atlantic sea cucumber (Cucumaria frondosa) and green urchin (Strongylocentrotus droebachiensis) processing plants were investigated. Results showed that sea cucumber viscera, urchin digestive tract (UDT) and non-commercial grade gonads contained 4.5–14.4% total proteins and 1.9–4.6% lipids (wet weight basis). They were rich in essential amino acids with values of about 36% of total amino acids. Samples were also rich in polyunsaturated fatty acids (44% of total fatty acids) and showed high content in EPA (∼17% of total fatty acids), but low DHA level (0.2%) was observed. In terms of vitamins content, UDT showed highest retinol and alpha-tocopherol contents when compared to other marine samples. Results showed that by-products from echinoderm processing plants contain various nutritious components with levels generally comparable even better than their edible counterparts, and efforts should be invested to valorise these biological wastes in a world context of rarefaction of nutritional marine resources.