Boric acid (BA) is an essential nutrient for plants and many organisms, but it has become an environmental contaminant because of widespread use. Pesticide and its compounds are a serious threat to aquatic organisms. This study was carried out to determine the histopathological effects of acute exposure to BA concentrations in rainbow trout. The fish were exposed to 102 and 103 mg/L concentrations of BA. Tissues were sampled at 6, 12, 24, 48 and 96 h. Histopathological alterations occurring in tissues were common in both doses of BA. Gill tissues showed lamellar oedema, cellulary infiltration, lamellar disorganization, degenerative changes and lamellar thickening. Kidneys had glomerular oedema and glomerulonephritis, degeneration of the tubulary epithelium, interstitial fibrosis and a hyaline cast within the tubular lumens. Muscle tissues displayed interstitial oedema and degenerative and atrophic changes to varying degrees in the myofibrils. Our study shows that BA can be toxic for rainbow trout and cause histopathological damage in fish tissue.