We present a catalogue of ∼3000 submillimetre sources detected (≥3.5σ) at 850 μm over ∼5 deg2 surveyed as part of the James Clerk Maxwell Telescope (JCMT) SCUBA-2 Cosmology Legacy Survey (S2CLS). This is the largest survey of its kind at 850 μm, increasing the sample size of 850 μm selected submillimetre galaxies by an order of magnitude. The wide 850 μm survey component of S2CLS covers the extragalactic fields: UKIDSS-UDS, COSMOS, Akari-NEP, Extended Groth Strip, Lockman Hole North, SSA22 and GOODS-North. The average 1σ depth of S2CLS is 1.2 mJy beam−1, approaching the SCUBA-2 850 μm confusion limit, which we determine to be σc ≈ 0.8 mJy beam−1. We measure the 850 μm number counts, reducing the Poisson errors on the differential counts to approximately 4 per cent at S850 ≈ 3 mJy. With several independent fields, we investigate field-to-field variance, finding that the number counts on 0.5°–1° scales are generally within 50 per cent of the S2CLS mean for S850 > 3 mJy, with scatter consistent with the Poisson and estimated cosmic variance uncertainties, although there is a marginal (2σ) density enhancement in GOODS-North. The observed counts are in reasonable agreement with recent phenomenological and semi-analytic models, although determining the shape of the faint-end slope (S850 10 mJy there are approximately 10 sources per square degree, and we detect the distinctive up-turn in the number counts indicative of the detection of local sources of 850 μm emission, and strongly lensed high-redshift galaxies. All calibrated maps and the catalogue are made publicly available. ; The authors thank M. Bethermin for supplying the SCUBA-2 850 µm number count predictions. The James Clerk Maxwell Telescope is now operated by the East Asian Observatory on behalf of The National Astronomical Observatory of Japan, Academia Sinica Institute of Astronomy and Astrophysics, the Korea Astronomy and Space Science Institute, the National Astronomical Observatories of China and the Chinese Academy of Sciences (grant no. XDB09000000), with additional funding support from the Science and Technology Facilities Council of the United Kingdom and participating universities in the United Kingdom and Canada. The data presented in this paper were taken as part of Program ID MJLSC02. It is a pleasure to thank the entire staff of the JCMT for their superb support throughout the S2CLS campaign. Special thanks is due to Iain Coulson, Jessica Dempsey, Jim Hoge, Harriet Parsons, Callie Matulonis, William Montgomerie and Holly Thomas. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. This work used the DiRAC Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment was funded by BIS National E-infrastructure capital grant ST/K00042X/1, STFC capital grant ST/H008519/1, and STFC DiRAC Operations grant ST/K003267/1 and Durham University. DiRAC is part of the National E-Infrastructure. JEG is supported by a Royal Society University Research Fellowship. IRS, AMS, JMS, ALRD, DMA, ACE and CGL acknowledge support from ST/L00075X/1. IRS also acknowledges support from the ERC Advanced Grant DUSTYGAL (321334) and a Royal Society Wolfson Merit Award. JSD acknowledges the support of the European Research Council through the award of an Advanced Grant. RJI acknowledges support from ERC in the form of the Advanced Investigator Programme COSMICISM (321302). AK acknowledges support by the Collaborative Research Council 956, sub-project A1, funded by the Deutsche Forschungsgemeinschaft (DFG). KK acknowledges supports from the Swedish Research Council. ; Peer-reviewed ; Publisher Version