A 6.8 nb-1 sample of pp collision data collected under low-luminosity conditions at s=7TeV by the ATLAS detector at the Large Hadron Collider is used to study diffractive dijet production. Events containing at least two jets with pT>20GeV are selected and analysed in terms of variables which discriminate between diffractive and non-diffractive processes. Cross sections are measured differentially in δηF, the size of the observable forward region of pseudorapidity which is devoid of hadronic activity, and in an estimator, ξ~, of the fractional momentum loss of the proton assuming single diffractive dissociation (pp→pX). Model comparisons indicate a dominant non-diffractive contribution up to moderately large δηF and small ξ~, with a diffractive contribution which is significant at the highest δηF and the lowest ξ~. The rapidity-gap survival probability is estimated from comparisons of the data in this latter region with predictions based on diffractive parton distribution functions.