Biodegradable sizing agents from triethanolamine (TEA)-modified soy proteins have been developed to replace polyvinyl alcohol (PVA) sizes for high-speed weaving. Previously, we found soy protein showed better sizing performance properties, compared to wheat gluten and keratin. However, for high-speed weaving, sizing films from raw soy protein still lacked flexibility and adhesiveness. Introduction of TEA considerably improved the film properties and sizing properties of soy protein. Compared to PVA, TEA-modified soy protein size had remarkably better biodegradability, improved capability of hair coverage, comparable film flexibility, and adhesion to fibers. TEA-modified soy protein had weaving efficiencies in industrial-scale tests similar to commercial PVA sizes. In general, TEA-modified soy protein could be an effective substitute for PVA to promote sustainability in the textile sizing industry.