The QCD axion is a good dark matter candidate. The observed dark matter abundance can arise from misalignment or defect mechanisms, which generically require an axion decay constant f a ∼ Oð10 11 Þ GeV (or higher). We introduce a new cosmological origin for axion dark matter, parametric resonance from oscillations of the Peccei-Quinn symmetry breaking field, that requires f a ∼ ð10 8 -10 11 Þ GeV. The axions may be warm enough to give deviations from cold dark matter in large scale structure.