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Abstract To investigate the effect and mechanism of chitosan nanoparticles (CSNPs) on the germination and seedling growth of wheat (Triticum aestivum L.), we conducted systematic research on the impact of different concentrations (1–100 μg/mL) of CSNPs and chitosan (CS). The result of energy-dispersive spectroscopy (EDS) and confocal laser scanning microscopy (CLSM) showed that adsorption of CSNPs on the surface of wheat seeds was higher than that of CS. CSNPs had growth promoting effect at a lower concentration (5 μg/mL) compared with CS (50 μg/mL). In addition, the application of 5 μg/mL CSNPs induced the auxin-related gene expression, accelerated indole-3-acetic acid (IAA) biosynthesis and transport, and reduced IAA oxidase activity resulting in the increase of IAA concentration in wheat shoots and roots. The results suggest that CSNPs have positive effect on seed germination and seedling growth of wheat at a lower concentration than CS due to higher adsorption on the surface of wheat seeds. Highlights • CSNPs could promote growth at a lower concentration about 10 fold than CS. • CSNPs promote wheat growth by activating IAA signaling pathway. • The CSNPs adsorption is stronger than that of CS on the surface of wheat seed by EDS and CLSM. [ABSTRACT FROM AUTHOR]