Triclosan (TCS), a widely used antimicrobial, has emerged as a concerning aquatic pollutant, especially post-COVID-19 due to increased disinfectant use. The study aimed to investigate the toxic impacts of TCS on Labeo rohita, revealing a 96-h LC of 0.742 mg/L. Well-acclimatized fish were categorised into three groups: one control and two treatment (T1 and T2) that were exposed to 1/10th and 1/5th of 96-h LC i.e., 0.0742 mg/L and 0.148 mg/L, respectively for 6 weeks under semi-static condition. Significantly (p < 0.05) elevated extents of reactive oxygen species (ROS) in the liver indicated enhanced oxidative stress. The activities of enzymatic antioxidants viz. superoxide dismutase (SOD) and catalase (CAT) were significantly (p < 0.05) increased while the reduced glutathione (GSH) levels were significantly (p < 0.05) decreased in a dose- and duration-dependent manner. A significant (p < 0.05) increase was observed in the lipid peroxidation (LPO) rate, which coincided with disruptions in the histological structure of the liver. Significant (p < 0.05) induction in Micronuclei frequency validated genomic instability. Furthermore, Endoplasmic Reticulum (ER) stress was marked by the increased intracellular calcium levels and elevated expression of grp78, chop, atf4, perk, eIF2α and gadd34 in TCS-exposed groups. Moreover, the consequent activation of bax, caspase-3, caspase-9, apaf-1 and the regulation of bcl-2 due to ER-stress, ultimately resulted in apoptosis of hepatocytes. Pearson correlation matrix and Principal Component Analysis (PCA) further underscored key interactions among parameters. Our findings demonstrate that the TCS, at its sub-lethal concentrations, is causing severe anomalies and even cell-death via triggering ER-stress-mediated apoptotic pathway in the fish liver.