Brain-derived amyloid-beta (A beta) dimers are associated with Alzheimer's disease (AD). However, their covalent nature remains controversial. This feature is relevant, as a covalent cross-link has been proposed to make brain-derived dimers (brain dimers) more synaptotoxic than A beta monomers and would also make them suitable candidates for biomarker development. To resolve this controversy, we here present a three-step approach. First, we validated a type of synthetic cross-linked A beta (CL A beta) dimers, obtained by means of the photoinduced cross-linking of unmodified proteins (PICUP) reaction, as well-defined mimics of putative brain CL A beta dimers. Second, we used these PICUP CL A beta dimers as standards to improve the isolation of brain A beta dimers and to develop state-of-the-art mass spectrometry (MS) strategies to allow their characterization. Third, we applied these MS methods to the analysis of brain A beta dimer samples allowing the detection of the CL [A beta(6-16)](2) peptide comprising a dityrosine cross-link. This result demonstrates the presence of CL A beta dimers in the brains of patients with AD and opens up avenues for establishing new therapeutic targets and developing novel biomarkers for this disease.