Abstract: In recent years, microplastics have garnered widespread attention as emerging environmental contaminants. In natural environments, microplastics often coexist with traditional pollutants, forming combined exposure scenarios that can alter their environmental impacts. This study investigates the effects of single exposure to polystyrene nanoplastics (PSNPs) at concentrations ranging from 10 mg/L to 500 mg/L, as well as combined exposure to PSNPs (100 mg/L) and cadmium (Cd) (30 mg/L), on the germination and metabolism of rice seeds (Oryza sativa L.). The key findings are as follows: (1) PSNPs at concentrations of 10–500 mg/L showed no significant inhibitory effect on seed germination rate (GR > 85% across all test concentration range), but demonstrated substantial accumulation in seeds, roots, and shoots, with root PSNP concentrations being orders of magnitude higher than those in other tissues. (2) Compared to the control group, PSNP exposure reduced the dry biomass of roots and shoots by 50%, while exhibiting minimal effects on root and shoot length. (3) Combined exposure experiments revealed that the root and shoot lengths in the co-exposure group were significantly greater than those in the Cd-alone group, suggesting that PSNPs mitigate Cd phytotoxicity through adsorption mechanisms. (4) Metabolomic analysis showed that both single exposure (Cd group, PSNPs group) and combined exposure (CT group) all significantly changed metabolic pathways during rice germination. (5) Cd primarily disrupted carbohydrate metabolism, amino acid metabolism, and flavonoid biosynthesis, while PSNPs predominantly affected amino acid metabolism. The combined contamination resulted in more complex metabolic perturbations, including carbohydrate metabolism, lipid metabolism, and riboflavin metabolism. (6) The addition of PSNPs enhanced glycolytic pathways and osmotic regulation, thereby alleviating Cd-induced oxidative stress. This study demonstrates that while PSNPs alone have limited impacts on rice germination, their co-exposure with Cd mitigates Cd toxicity through metabolic regulatory mechanisms. These findings provide novel insights into plant adaptive responses to combined contaminant exposure.