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The production of extra virgin olive oil generates significant amounts of olive mill waste, whose disposal leads to severe environmental impacts, especially due to the high content of phenolic compounds. In this study, a pomace phenolic extract composed of hydroxytyrosol, tyrosol, verbascoside, and oleuropein was obtained from the liquid fraction of two-phase olive pomace and explored for its antimicrobial properties and potential as plant immunostimulants. The olive pomace extract exhibited a broad range of antimicrobial activity against important phytopathogens, including the bacteria Xylella fastidiosa, Pseudomonas syringae, and Pectobacterium carotovorum, as well as the fungi Colletotrichum graminicola, Fusarium graminearum, and Botrytis cinerea. The extract induced key features of plant innate immunity in Arabidopsis seedlings, including hydrogen peroxide production, phosphorylation of mitogen-activated protein kinase MAPK6, and upregulation of defence genes, such as CYP81F2, FRK1, and WRKY53, suggesting the activation of early signalling cascades leading to the production of indole glucosinolates and salicylic acid. The immune activation pathways induced by the phenolic extract did not always match those triggered by well-known oligogalacturonide elicitors. Notably, pretreatment of adult Arabidopsis and tomato plants with the phenolic compounds-enriched extract primed responses and enhanced their resistance against B. cinerea and P. syringae. Our findings demonstrate the potential to upcycle two-phase olive pomace into plant protectants, offering a promising alternative to reduce reliance on chemically synthesized pesticides in integrated pest management programs.