Mussel protein can serve as a strong and water-resistant adhesive, but is expensive and not readily available. Soy protein is inexpensive, abundant, and readily available, but is a poor adhesive. Mussel protein contains a high amount of mercapto-containing cysteine. This study revealed that increasing the free mercapto group content in soy protein could greatly increase the strength and water-resistance of wood composites bonded with the modified soy protein.