The increasing use of cobalt nanoparticles (CoNPs) in industrial and biomedical applications raises concerns about their environmental fate and impact on aquatic ecosystems. In this study, we evaluated the phytotoxicity and phytoremediation potential of CoNPs using Lemna minor, a macrophyte known for its metal accumulation capacity. Specifically, we investigated CoNP-induced oxidative stress, physiological responses, and nanoparticle uptake mechanisms. L. minor plants were exposed to CoNPs at concentrations ranging from 0.1 to 20,000 µg L⁻¹ for seven days. Growth rates, photosynthesis, and respiration were measured, while oxidative stress markers (H₂O₂ and MDA) and antioxidant enzyme activities (SOD, APX, and CAT) were analyzed. Cobalt accumulation in plant tissues was quantified using ICP-MS, and transmission electron microscopy (TEM) was used to assess ultrastructural changes and intracellular cobalt localization. Our results showed that CoNP exposure led to significant oxidative stress and metabolic impairment at concentrations ≥1000 µg L⁻¹, reducing photosynthesis by 85% and growth by 77%. TEM analyses revealed that CoNPs were internalized by L. minor and localized in chloroplasts, causing thylakoid disorganization and plastoglobuli accumulation. Interestingly, CoNPs were found in different subcellular locations than Co²⁺ ions, suggesting distinct toxicity mechanisms. Despite these effects, L. minor removed over 99% of CoNPs and Co²⁺ from the medium, highlighting its potential for nanoparticle remediation. This study provides novel insights into the uptake and impact of CoNPs in aquatic plants, reinforcing the role of L. minor as an effective phytoremediator in nanoparticle-contaminated environments. Future research should explore long-term accumulation dynamics and optimize phytoremediation strategies for sustainable nanoparticle management.