Using formic acid as a cosolvent and saturated sodium sulfate as a coagulation bath, soy protein/poly(vinyl alcohol) (PVA) blend fibers were prepared using wet-spinning approaches. The structure and mechanical, thermal, and water-uptake properties of the spun fibers were investigated. Morphological analysis with polarized optical microscopy (POM) and scanning electron microscopy (SEM) showed that spun PVA or blend fibers were composed of nanoparticles and exhibited a porous morphology. Blend fibers exhibited only one glass transition temperature in differential scanning calorimetry (DSC) thermograms due to high compatibility between the two components. The best mechanical strength and thermal stability were achieved when 70% PVA was composited with soy protein. This was thought to be due to the effects of cross-linking and hydrogen bonding between functional groups of soy protein and PVA hydroxyl groups.