With the increasing demand for protein worldwide, marine shellfish protein has attracted more and more attention because of its large yield and high nutritional value. To date, the output of shellfish aquaculture in China has continued to grow. The sustainable development of marine shellfish protein has become a hot topic. To improve the deep-processing level of marine shellfish proteins, the molecular composition and characteristics of marine shellfish proteins were reviewed in this chapter. In particular, the marine shellfish protein composition, amino acid composition, temperature stability, pH stability, and ion-binding properties are involved. These are expected to provide new ideas for the practical application of marine shellfish proteins in the medical and food industries.