The ubiquitous occurrence of ibuprofen (IBU) in aquatic ecosystems necessitates an assessment of its ecotoxicological effects. IBU's toxicity can vary with pH fluctuations, as its ionizable state significantly influences its environmental fate and bioaccumulation potential. This study examined the pH-dependent toxicity of IBU on Chlorella vulgaris by evaluating its oxidative stress response and metabolomic profile. Algae were exposed to IBU from environmentally relevant concentrations (1-10 μg/L) to sublethal concentrations (0.1-20 mg/L) under three pH levels (6, 7, and 8), using n = 3 (phenotypic) and n = 5 (metabolomics) replicates. Metabolomic quality was validated by tight clustering of pooled QC samples. Integrating data on algal growth, chlorophyll content, oxidative stress markers, and metabolic profiles confirmed the pH-dependent toxicity of IBU, whose increased toxic effects are likely due to an enhanced fraction of unionized IBU at lower pH. The influence of pH on IBU toxicity was substantiated by alterations in metabolite levels, including Adenosine diphosphate, uridine 5'-diphosphate, uridine, and cytidine. Furthermore, the mechanisms underlying IBU's molecular toxicity concerning pH variable were analyzed based on differentially expressed intracellular metabolites with fold-change (FC) > 2 or <0.5 (p < 0.05), thereby elucidating IBU-induced biological pathways such as riboflavin metabolism, the citrate cycle, and pyruvate metabolism, which align with the observed oxidative stress responses. Our integrative metabolomic and oxidative-stress study underscores that mild acidification can enhance ibuprofen's sublethal effects on the algal metabolome.Copyright © 2025. Published by Elsevier Inc.