Silver nanoparticles (nAg) raise ecotoxicological concerns due to their increasing production, application, and inevitable release into aquatic environments. In freshwater, nAg aggregate and partially dissolve, coexisting with dissolved silver (Ag⁺ ). Diatoms, key phytoplankton contributing to global silicon and carbon cycles, are particularly relevant for assessing silver toxicity, yet the metabolic impacts of Ag⁺ and nAg remain poorly understood. Here, we combined metabolomics and physiological assays to investigate the effects of short-term (2 h), sublethal exposures of Ag⁺ (0.01 and 0.02 mg L^-1 ) and nAg (0.1 and 0.3 mg L^-1 ) on the freshwater diatom Cyclotella meneghiniana . Both silver forms strongly affected the diatom metabolism, with notable alterations in amino acid, polyamine, glutathione, and nucleotide metabolism, as well as the tricarboxylic acid (TCA) cycle. These metabolic shifts were dependent on the silver form and concentration, and correlated with intracellular silver accumulation. Notably, nAg exerted stronger and more specific effects on the TCA cycle, glutathione metabolism, and polyamine synthesis than Ag⁺ . Physiologically, both nAg and Ag⁺ impaired photosystem performance, but only nAg induced enhanced reactive oxygen species (ROS) production and altered carbonic anhydrase activity. This study provides the first insights into both shared and distinct metabolomic responses of diatoms to short-term exposure to Ag⁺ and nAg, highlighting the greater specificity and potency of nAg in disrupting key metabolic and physiological processes.