Akin to mammalian extracellular fluids, the plant apoplastic fluid (APF) contains a unique collection of proteins, RNAs, and vesicles that drive many physiological processes ranging from intercellular communications and cell wall remodeling to defense against abiotic and biotic challenges. Using an improved method to enrich for the ArabidopsisAPF from leaves combined with deep proteomic analyses, we better defined its composition and discovered that the APF also contains numerous proteases and functional proteasomes central to both ubiquitin-mediated and independent proteolysis. Evidence for ex-proteasomes includes; morphological detection by electron microscopy, proteasome-specific activity assays sensitive to selective inhibitors, immunoblotting with proteasome-specific antibodies, immunodepletion assays with seedlings expressing Flag-tagged proteasomes, and tandem mass spectrometry of APF showing enrichment of 20S core particle subunits along with several 19S regulatory particle subunits. Given the importance of the apoplast to host-pathogen interactions, we also present evidence that ex-proteasomes promote protection by helping generate microbe-associated molecular patterns (MAMPs) from invading pathogens, which trigger host-mediated, reactive oxygen species (ROS) defense responses. Like the flagellin-derived flg22 MAMP, we could stimulate ROS burst by infiltrating Arabidopsis leaves with APF incubated with either the Pseudomonas syringae pathogen or its enriched flagellin but via a process suppressed by the proteasome-specific inhibitor bortezomib, implying that ex-proteasomes work upstream of flg22 sensing. Collectively, we provide a deep catalog of apoplast-localized proteins and intriguing evidence that plants secrete proteasomes for biotic defense.