Plant immunity involves complex regulatory mechanisms that mediate the activation of defense responses against pathogens. Protein degradation via ubiquitination plays a crucial role in modulating these defenses, with E3 ubiquitin ligases functioning as central regulators. This study investigates the role of SlATL2, an ARABIDOPSIS TÓXICOS EN LEVADURA (ATL)-type E3 ubiquitin ligase localized in the plasma membrane, in the immune response of tomato plants against pv. () DC3000. Our findings demonstrate that SlATL2 expression is induced upon DC3000 infection and treatment with defense hormones salicylic acid and jasmonic acid. Functionally, SlATL2 negatively regulates immune responses, impairing resistance to DC3000 and suppressing flg22-triggered immunity. In addition, SlATL2 limits pathogen-induced reactive oxygen species and callose accumulation by targeting the COP9 signalosome subunit 5a (SlCSN5a), a key positive regulator of tomato defense responses against DC3000. This interaction, which occurs via the N-terminal residue of SlATL2, results in the ubiquitination and 26S proteasomal degradation of SlCSN5a, thereby suppressing SA-dependent expression of defense response genes associated and limiting reactive oxygen species production. This work sheds light on the molecular mechanism through which the E3 ubiquitin ligase SlATL2 attenuates tomato immune responses by targeting a COP9 signalosome subunit for degradation. These discoveries deepen our insights into the post-translational mechanisms governing plant immune responses and provide fresh opportunities to bolster crop resistance against bacterial pathogens.