<p>The antioxidant defense mechanisms that shield eukaryotes from oxidative stress depend heavily on the enzyme catalase (CAT). These proteins, which are found in nearly all living creatures, perform crucial functions in regulating how plants react to biotic and abiotic stimuli by regulating the breakdown of H₂O₂. CAT is encoded by a small gene family in plants. In the current study, the first catalase gene from oat (<i>Avena sativa</i> L.) designated <i>AsCAT1</i> was isolated and characterized. The corresponding AsCAT1 protein has 492 amino acids and presented a significant similarity with other catalase proteins in subfamily 1, according to phylogenetic study. A peroxisomal targeting signature (PTS1), as shown for other catalase proteins, is present at the C-terminal portion of AsCAT1 which confers a peroxisomal localization of this protein. AsCAT1 protein has a catalytic activity that could be stimulated by different cations. The expression of AsCAT1 protein in bacterial cells conferred tolerance to some abiotic stresses (NaCl, sorbitol, and LiCl). <i>AsCAT1</i> is highly expressed in leaves, but its expression is low in roots as previously shown for other monocotyledonous plants. Interestingly, <i>AsCAT1</i> gene expression is upregulated in response to a variety of stimuli, including hormonal, osmotic, salt, and heavy metal exposures. Our data strongly suggest that <i>AsCAT1</i> is a crucial gene implicated in oat to aid this species in fending off environmental challenges. Such results help in further understanding the functions of catalase proteins in monocotyledonous plants in general and oat in particular.</p>