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Many bacteria, including the major human pathogen Pseudomonas aeruginosa, are naturally found in multicellular, antibiotic-tolerant biofilm communities, in which cells are embedded in an extracellular matrix of polymeric molecules. Cell–cell interactions within P. aeruginosa biofilms are mediated by CdrA, a large, membrane-associated adhesin present in the extracellular matrix of biofilms, regulated by the cytoplasmic concentration of cyclic diguanylate. Here, using electron cryotomography of focused ion beam–milled specimens, we report the architecture of CdrA molecules in the extracellular matrix of P. aeruginosa biofilms at intact cell–cell junctions. Combining our in situ observations at cell–cell junctions with biochemistry, native mass spectrometry, and cellular imaging, we demonstrate that CdrA forms an extended structure that projects from the outer membrane to tether cells together via polysaccharide …