Cottonseed protein is an abundant by-product from the cotton industry, yet its severe aggregation and poor solubility under neutral physiological conditions greatly restrict its high-value utilization in food and nutritional formulations. In this work, we constructed a green pH-shifted self-assembly strategy combining cottonseed protein and natural shellac resin to fabricate colloidal nanocomplexes. The protein aggregates were first dissociated under alkaline conditions, followed by neutralization-triggered co-assembly between cottonseed protein and shellac. Under optimal conditions, the neutral solubility of cottonseed protein was dramatically increased from 6.8% to 94.5%, with narrower particle size distribution and markedly enhanced colloidal stability. Multi-spectroscopic and microscopic characterizations demonstrated that hydrogen bonding and hydrophobic interactions dominated the formation of the binary complex. When applied as a delivery matrix, the assembled nanoparticles efficiently encapsulated curcumin, ameliorating its poor water solubility, photodegradation and thermal instability. In simulated gastrointestinal digestion, the system exhibited pH-responsive release behavior and improved curcumin bioaccessibility. This study clarifies the solubilization mechanism of the cottonseed protein-shellac complex and provides a clean-label, sustainable route for valorizing cottonseed protein and delivering lipophilic bioactive compounds in functional food systems.