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Tomato (<i>Solanum lycopersicum</i> L.) is among the most important vegetable crops worldwide. Early blight disease, caused by <i>Alternaria solani</i>, is a destructive foliar disease of tomato and other Solanaceae species. Herein, we investigated the <i>in vitro</i> antifungal properties of gallic acid and two of its derivatives (syringic and pyrogallic acids) against <i>A. solani</i> during 2019 and 2020 seasons. The physiological and biochemical effects of these compounds on infected tomato plants were also investigated using the whole plant bioassay. The <i>in vitro</i> investigation showed that all tested compounds showed fungistatic action and inhibited the mycelial radial growth of <i>A. solani</i> in a dose-dependent manner. In two separate pot-experiments, those compounds efficiently suppressed the development of the disease symptoms and area under disease progress curve (AUDPC), without any phytotoxic effects on the treated tomato plants. Additionally, all tested compounds positively enhanced the biochemical traits of treated plants including the chlorophyll content, the total soluble phenolics, the total soluble flavonoids, and the enzymatic activities of catalase, peroxidase, and polyphenol oxidase during 2019 and 2020 seasons. Moreover, the treatment with gallic acid and its derivatives significantly increased all yield components of <i>A. solani</i>-infected tomato plants such as the total number of flowers and fruits, and the fruit yield for each tomato plant in both experiments. Considering the fungitoxicity of phenolic acids against <i>A. solani</i> with no phytotoxicity on treated tomato plants, we believe that gallic acid and its derivatives might be a sustainable eco-friendly control strategy to reduce the usage of chemical fungicides partially or entirely against <i>A. solani</i> particularly, and fungal diseases in general.