<p>Drought is an abiotic stress factor affecting crop yield and quality worldwide. Cerium oxide nanoparticles (CeO₂ NPs) or nanoceria can alleviate drought stress by modulating photosystem and gene expression. This study aims to investigate the ameliorative effects of nanoceria and possible underlying mechanisms in barley (<i>Hordeum vulgare</i> L.).&#xa0;For this purpose, barley seedlings were treated with nanoceria (50–800 mg L^-1, ~ 20&#xa0;nm in diameter) under well-watered and polyethylene glycol (PEG; 10%)-induced water deficit conditions. Plant growth, biomass, pigment content, photosynthetic performance, and expression of photosystem marker genes were assessed after three weeks of treatment.&#xa0;PEG treatment significantly (<i>P</i> &lt; 0.05) reduced growth, biomass, photosynthetic pigments (chlorophyll-a, chlorophyll-b, carotenoids), and photosystem performance (electron transport rate, maximum potential quantum efficiency of Photosystem II, and quantum yield of Photosystem II). CeO₂ NPs at 50 and 100 mg L^-1 doses significantly enhanced root and shoot length, as well as biomass. These concentrations remarkably mitigated PEG-induced reductions in growth, biomass, and photosynthetic indices. On the other side, CeO₂ NP at higher concentrations (&gt; 200 mg L^-1) diminished this positive effect, indicating a biphasic response. Gene expression analysis revealed that while PEG treatment alone upregulated photosystem (<i>petA</i>, <i>psaA</i>, <i>psbA</i>, <i>BCA</i>) and stress (<i>Hsp90</i>) genes, CeO₂ NPs with PEG treatment (200 mg L^-1) decreased their expression.&#xa0;These findings demonstrate that nanoceria helps to ameliorate drought stress by enhancing growth and photosynthetic activity and modulating gene expression in barley.</p>