Searches are performed for both promptlike and long-lived dark photons, A, produced in proton-proton collisions at a center-of-mass energy of 13 TeV. These searches look for A→μ+μ-decays using a data sample corresponding to an integrated luminosity of 5.5 fb-1 collected with the LHCb detector. Neither search finds evidence for a signal, and 90% confidence-level exclusion limits are placed on the γ-A kinetic mixing strength. The promptlike A search explores the mass region from near the dimuon threshold up to 70 GeV and places the most stringent constraints to date on dark photons with 214<m(A)â‰740 MeV and 10.6<m(A)â‰30 GeV. The search for long-lived A→μ+μ-decays places world-leading constraints on low-mass dark photons with lifetimes O(1) ps.