We present a 1.1mm wavelength imaging survey covering 0.3deg^2^ in the COSMOS field. These data, obtained with the AzTEC continuum camera on the James Clerk Maxwell Telescope, were centred on a prominent large-scale structure overdensity which includes a rich X-ray cluster at z>>0.73. A total of 50mm-galaxy candidates, with a significance ranging from 3.5 to 8.5sigma, are extracted from the central 0.15deg^2^ area which has a uniform sensitivity of 1.3mJy/beam. 16 sources are detected with S/N>=4.5, where the expected false-detection rate is zero, of which a surprisingly large number (9) have intrinsic (deboosted) fluxes >=5mJy at 1.1mm.