Gram-negative bacteria are surrounded by a cell envelope that comprises an outer membrane (OM) and an inner membrane that, together, delimit the periplasmic space, which contains the peptidoglycan (PG) sacculus. Covalent anchoring of the OM to the PG is crucial for envelope integrity in Escherichia coli. When the OM is not attached to the PG, the OM forms blebs and detaches from the cell. The Braun lipoprotein Lpp¹ covalently attaches OM to the PG but is present in only a small number of γ-proteobacteria; the mechanism of OM–PG attachment in other species is unclear. Here, we report that the OM is attached to PG by covalent cross-links between the N termini of integral OM β-barrel-shaped proteins (OMPs) and the peptide stems of PG in the α-proteobacteria Brucella abortus and Agrobacterium tumefaciens. Cross-linking is catalysed by l,d-transpeptidases and attached OMPs have a conserved alanyl-aspartyl motif at their N terminus. Mutation of the aspartate in this motif prevents OMP cross-linking and results in OM membrane instability. The alanyl-aspartyl motif is conserved in OMPs from Rhizobiales; it is therefore feasible that OMP–PG cross-links are widespread in α-proteobacteria.