A search for the rare η→μ+μ-μ+μ- double-Dalitz decay is performed using a sample of proton-proton collisions, collected by the CMS experiment at the CERN LHC with high-rate muon triggers during 2017 and 2018 and corresponding to an integrated luminosity of 101 fb-1. A signal having a statistical significance well in excess of 5 standard deviations is observed. Using the η→μ+μ- decay as normalization, the branching fraction B(η→μ+μ-μ+μ-)=[5.0±0.8(stat)±0.7(syst)±0.7(B2μ)]×10-9 is measured, where the last term is the uncertainty in the normalization channel branching fraction. This work achieves an improved precision of over 5 orders of magnitude compared to previous results, leading to the first measurement of this branching fraction, which is found to agree with theoretical predictions.