We present evidence for a strong relationship between galaxy size and environment for the quiescent population in the redshift range 1 < z < 2. Environments were measured using projected galaxy overdensities on a scale of 400 kpc, as determined from ~96 000 K-bandselected galaxies from the UKIDSS Ultra Deep Survey (UDS). Sizes were determined from ground-based K-band imaging, calibrated using space-based CANDELS HST observations in the centre of the UDS field, with photometric redshifts and stellar masses derived from 11-band photometric fitting. From the resulting size-mass relation, we confirm that quiescent galaxies at a given stellar mass were typically ~50 per cent smaller at z ~ 1.4 compared to the present day. At a given epoch, however, we find that passive galaxies in denser environments are on average significantly larger at a given stellar mass. The most massive quiescent galaxies (Mz.ast; >2×1011M⊙) atz>1 are typically 50 per cent larger in the highest density environments compared to those in the lowest density environments. Using Monte Carlo simulations, we reject the null hypothesis that the size-mass relation is independent of environment at a significance >4.8s for the redshift range 1< z<2. In contrast, the evidence for a relationship between size and environment is much weaker for star-forming galaxies. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Published by Oxford University Press on behalf of the Royal Astronomical Society.