Measurements of fiducial and differential cross sections are presented for Higgs boson production in proton-proton collisions at a centre-of-mass energy of 8TeV. The analysis is performed in the Higgs diphoton decay channel using 20.3/fb of data recorded by the ATLAS experiment at the CERN Large Hadron Collider. The signal yields are corrected for the effects of detector inefficiency and resolution. Differential cross sections are also presented, as a function of variables related to the diphoton kinematics and the jet activity produced in the Higgs boson events. The observed spectra are statistically limited but broadly in line with the theoretical expectations. Note on reinterpretation: Since the background is subtracted from a fit, any BSM signal would presumably also have beed subtracted. Therefore should not be used to constrain BSM signals unless they comes from a genuine excess of SM Higgs->gammagamma.