Trifluoroacetate (TFA) has been assumed to be an innocuous counterion (to cationic amino acid side chains) present in countless synthetic bioactive peptides and a few FDA-approved therapeutics. We show here that TFA is in fact bioactive and causes dramatic biological effects in multiple strains of mice and cultured human and rat liver cells. In high-fat diet (HFD)-fed low-density lipoprotein receptor-null (LDLr -/- ) mice, TFA reduces the levels of plasma cholesterol, triglycerides, and the development of atherosclerotic lesions following either oral or intraperitoneal administration. These physiological effects were observed with TFA alone, or with TFA present as a counterion of a variety of short, unrelated synthetic peptide sequences. Mechanistic investigations including RNA-seq, confocal microscopy, western blotting, metabolomics, proteomics, pharmacokinetics, and biochemical assays indicated that TFA induces peroxisome proliferation by activating peroxisome proliferator-activated receptor (PPAR)-alpha. We confirmed that TFA also caused peroxisome proliferation and downstream phenotypic effects in cultured human and rat liver cells, wild-type C57/Bl mice, and apolipoprotein E-null (apoE -/- ) mice, leading to anti-atherosclerotic effects in the latter strain. Given that TFA is a counterion in many peptides employed in early research and development settings, these findings raise the possibility that TFA may be confounding or contributing to phenotypic changes observed in many studies involving peptides. Although our studies suggest that TFA or its analogues might have therapeutic applications, it should be noted that TFA is also a persistent environmental contaminant that is found at high levels in humans relative to other polyfluoroalkyl substances (PFAS), and is a major metabolite following treatment of patients with common inhaled anesthetics, suggesting that the biological effects reported here could have other implications for human health.